Resolution changes to 4:3 and its not changeable anylonger

So basically, I tried out every solution I found. None of them worked. I’m pretty new to Ubuntu and a few days after my first installation of Ubuntu 16.04 this problem first happened. Searched the internet but with no luck. So I tried Ubuntu 14.04 (2nd computer, its quite old). It worked fine for one day, the next day it started in 4:3 res again. I cant change it in the Display- Settings. And I dont really want to reinstall Ubuntu every day :’) Any help is appreciated.
Thanks in advance.