Derailleur catches

[NBuzz83]

Hey,
Ive got a bike that when I try to shift to the highest gear (3) on the front Derailleur, the back Derailleur catches and ends up being pulled forward and catches completely, stopping the pedals as well.
I'm not sure how to fix this.
N

[cny-man]

The most likely cause is that you are shifting to the large chainwheel in front while in the largest cog in the back AND the chain is too short. Other than that need a better explanation (what rear cog are you in, when did the problem start and was there any incident or repair procedure just prior to that) - or post a video of the problem.