There are 3 reasons you might lose rotor RPM and therefore lift.
1) You've flown off the edge of the map/there's no terrain collision under you - Some locations around the outskirts of maps appear to have terrain, but actually have no collision, so the helicopter thinks it's at unlimited altitude and tries to cap your lift to bring you back to a normal level, which makes you plummet rapidly until you move over something that
does have collision. If you find these spots, please point them out to us for fixing.
2) If you've taken damage to your engine, your max rotor RPM will decrease and make it harder to stay airborne (similarly you will lose lift from hits to the rotor assembly which will also make it harder to keep in the air, but this is harder to visibly identify)
The main cause though, and the one that appears to be your issue in that screenshot (based on what I see on the screen and HUD):
3) You've raised the collective all the way and then rolled into a hard bank. What this does is first increase the rotor drag to maximum, which starts to slow the rotor straight away, then blasted the underside of the rotor with a heap of air. Initially it tries to speed the rotor up, but the sudden excess of lift causes you to bleed off speed rapidly, while the rotor drag stays high, so the rotor can't stay up to speed and the drag then causes it to slow rapidly. So now you've lost a chunk of rotor RPM, meaning your lift is already down. You've got the helo rolled onto one side, so the lift component is angled and only a small percentage is actually able to fight gravity. And lastly, you've just bled off all your speed, so you're no longer getting much (if any) Effective Translational Lift (ETL). i.e. You've rapidly gone from a high energy, high lift state, to a low energy, low lift state, and you sink like a stone.
At this point you probably try to fight it by holding the collective at max and pulling the nose up to try and point rotor straight up and maximise your lift vector, but that's the worst thing you can do once you've entered this state. Now you're making it as hard for your rotor as possible to build back up to speed and you're taking active steps to prevent any chance of getting translational lift back, and so you accelerate downwards, or at the very least you decelerate very slowly, nowhere near enough to avoid an impact.
The last mentioned point there is the way the RPM suddenly comes back when you hit the ground. That's because in an instant you've snuffed out all airflow up into the rotor, the drag has dropped by a huge margin and the rotor is able to rapidly regain RPM. In fact, if you're bounced, now there's air moving the other way through the rotor, further reducing drag and speeding up the rotor even more.
But enough of what people do wrong - here's how to avoid it:
When you bank, you should avoid using full collective as much as possible unless you're trying to prevent an imminent collision with something like a cliff, a tree or another helicopter, or you intentionally want to cause a rapid fall (i.e. because you're being lit up by a SAM). You want to maintain as much kinetic energy as possible and that means gentle, gliding turns, and careful collective control.
If you do stuff it up and find yourself in the above situation, the way to recover is actually quite counter-intuitive; Lower the collective to 50% or thereabouts (even less can work too) and tip the nose into the direction of travel to build up speed. As you increase speed, you'll start getting ETL back, which will slow your descent, and the lower collective will let your rotor pick up speed more rapidly. Once the RPM has come up a bit it will already have started to boost your lift back again, and
then you can lift the nose and start to reapply collective, at which point you'll find you are able to pull up quite easily.
It's the same principle as performing an auto-rotation on engine failure.
As a last aside, you only get away with being able to pull these moves because I deliberately omitted two real-world effects that would kill you long before these things because a problem, on account of them being too unexpected and difficult for most gamers to deal with (as conditioned as we all are to video game physics). In reality, if you tried to pull either of the above manoeuvres, you'd be dead real quick, but we made some concessions to the fact that this is a game and not a full sim.
The first is rotor overspeed - if you try a hard bank at full collective in a real helicopter, you'll likely do some critical damage to the rotor assembly, if not rip it off entirely. That'd be 99% of our in-game pilots dead on their first attempt at turning, which isn't much fun.
The second is that if you yank on the collective while doing a rapid vertical descent, you would realistically enter a vortex ring state, where the RPM will climb back up but your descent will accelerate rapidly, as you descend through your own downwash and kill the effective angle of attack of the rotors by changing the direction of airflow.
But as I said, these were omitted as a concession to the fact that this is a game. The effects that
are included are to make the realism level of the helicopters roughly equivalent to the realism level of the infantry, i.e. Somewhat realistic but not a full sim.
And on a vaguely related note, here's a pro-tip for everyone: When you're waiting for the helo engine to wind up at the helipad, leave the collective at 0% until the rotor RPM reaches the green bar on the dial. Any collective you apply will add drag and that will slow down the acceleration of the rotor during startup (you can see that if you apply full collective, the rotor needle starts to fall behind the engine needle on the gauge), so leaving the collective all the way down will get you airborne faster than impatiently holding the "Up" key.