It could be mechanical or electrical. To narrow it down, power the game off and remove the glass. Move the flipper with your hand. Make sure it "flips" smoothly without binding and returns as normal. If it doesn't, lift the playfield and inspect for mechanical issues (loose links and screws / broekn parts / worn coil sleeves / etc).

If it's mechanically sound, then the problem is electrical. Could be as simple as a wire popping off from vibration and needing to be resoldered (it happens!), or a failed diode, or dirty / misaligned switch... to something a little more involved. But In my experience sudden flipper failure has usually been pretty simple and easily fixed.