Stupid Computer, What should i do?

Okay, so I got on Facebook and went to Picnik app. It usually works but until today, It said i needed to install the latest Adobe Flash Player. Well i tried to download it and everything, and a box popped up saying something about i need to log in on my administrators account so i can do it. Well i haven't been able to get on my main account, when i log into it its just a black screen, its been like this for a year or two. So, i have been on the 'Guest' account. What do i do?? HELP!