just downloaded krita and started a drawing, but I couldn't open the saved file after I exited the program =( What should I do?

If kritaui.dll is missing your install must have gotten corrupted. Which file did you download exactly? Was it an msi or a zip file?

It was a .msi
Should I just re download?

Just run the installer again and you should be fine.

I still can't open the file from the icon on my desktop, but I think something may be wrong with that file. I can open the drawing from hitting it in the open recent tab so I think I'm alright =)
also, do you know where the selection tools are? I want to select a small bit of my drawing and can't find them

Likely, the shortcut on the desktop points to the wrong place. The selection tools are in the bottom part of the toolbox.