Edit: The information in this post was out of date so has been deleted.
This issue has cropped up again recently. At the moment it looks like there may be problems on Steam's end. You may be able to find servers by using the Steam server browser instead of the in-game one - to do this right-click on the Steam tray icon and click Servers, and in the Servers tab make sure that you filter by Killing Floor and click Refresh. This should bring back a server list. If it doesn't then you can almost be sure that Steam is having more fundamental issues.
Also, Steam may have wiped the master server file for some people. Try hosting a listen server from the main menu, start playing and then disconnect, and then try finding servers again. Apparently this causes the file to be created.
This issue has cropped up again recently. At the moment it looks like there may be problems on Steam's end. You may be able to find servers by using the Steam server browser instead of the in-game one - to do this right-click on the Steam tray icon and click Servers, and in the Servers tab make sure that you filter by Killing Floor and click Refresh. This should bring back a server list. If it doesn't then you can almost be sure that Steam is having more fundamental issues.
Also, Steam may have wiped the master server file for some people. Try hosting a listen server from the main menu, start playing and then disconnect, and then try finding servers again. Apparently this causes the file to be created.
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