I had that happen if I accidentally hit the windows button in-game. When I alt-tabbed back, no FFB. I found 2 solutions:
1. Get a screwdriver and pop the Windows button off your keyboard (seriously!)
2. If you hit the Windows button, don't let it go; select another button such as the alt key, then let them both go, and the Windows button gets cancelled-out.
There is another solution:
if you lose ffb (after accidentally switch to another app or desktop), just land, hit refly, arming, select another plane, hit apply, hit arming again, select your plane, apply and fly. And ffb is back
Sometimes it is necessary to do this procedure twice