Alright, it's done. I figured the problem. Even though I erased (cleaned, not deleted) the Boot camp partition which had Ubuntu Linux on, the computer still started in the "BASH" black screen thing which I used to choose "Linux" from, but of course the OS is not there anymore. So I had to hold the alt (option) key every time I start, and that's what messed it up. When I chose Macintosh as start up disk, it worked fine.
Thanks for your help.