How to fix Err20

Error 20 on canon cameras means that the mirror is stuck. First, remove the battery and wait ~30 seconds before reinserting it and turning it on. If this does not resolve the error, you can attempt to follow something like this tutorial but this risks damaging the sensor. Unless you are very comfortable with disassmenbling prescision electronics, I would reccommend taking it to a professional camera repair store to fix the mirror.