Q: How can i change back the resolution for my external monitor.

Hello -PX-, 

After reviewing your post, it sounds like there is trouble with an unsupported resolution. I would recommend that you read this article, it may be able to help the issue.

Apple computers: Troubleshooting issues with video on internal or external displays - Apple Support

Display is unusable after changing to an unsupported resolution

If you experience a black screen, or otherwise have an unusable picture after changing to some resolutions, try resetting the NVRAM first.

 

If you continue to experience the issue, startup the Mac in Safe Mode. Delete the /Library/Preferences/com.apple.windowserver.plist file and restart the Mac.

Thanks for using Apple Support Communities.

Have a nice day,

Mario