Windows 8.1 black screen error?

I just downloaded the new update for Windows 8 and when i finished with all the setup work ( Skydrive etc.) the computer started booting up, nothing ever boots up. I know the computer is on because i see the usual neon lights it has but the screen is just black. I need help! I work on my computer every day! I went to a store and the employee told me that he has to delete every file from my computer. I have way too important stuff it it to let that happen. Help, please?!