Take me back, Sherman

OK. I’m a newbie, so please be kind. Using WordPress to create a blog for a class website. Started by using the default theme, easy enough to use, set it up so I am the Administrator, I log in and out, and all entries would have to go through me. All was well. Then I applied the Genman Theme, and now I can’t log in to create new entries. If you can explain how to fix the code, or even revert back to the default theme, I’d be very appreciative. Thanks.