Oliver, if your drone has calibration feature, re calibrate. Check the attached trouble shooting links,instruction and guides, Good luck
I hope this helped you out, if so let me know by pressing the helpful button.
http://www.droneflyers.com/2013/03/how-to-troubleshoot-your-ar-drone-1-0-and-2-0-after-a-crash/
https://community.parrot.com/t5/Airborne-Cargo-Drone/Left-front-propellor-not-working/td-p/118529
https://www.drone-forum.com/forum/viewtopic.php?t=1803