I'm not really sure what is happening. Things just started working offline and broke. Please see desc.

Ok, so recently, I thought it would be a good idea to download the multiplayer server while I had the single player server still installed so I could play with my friends online. This turned out to be so confusing and hard for me to comprehend, I just deleted all the multiplayer files from my C drive and told my friend he should probably do it( idk why he didn't do that in the first place, since he understands computers way better than I do). Next thing I know, trying to open minecraft reveals the same things, except the "Play" button is replaced with "Play offline" button. still brought me to the main screen, but when I click my world in single player it crashed and brought the following attachement: