I bought a used IMac with 10.7.5 os. The seller said she didn't have a password. It has tons of files, pics etc. that I wanted to delete but couldn't do so without resetting the password. A YouTube video said to press 'Restart', the Apple and 'R' button upon restart. It didn't say when to let go, so I might have held it too long. Now all I get is a grey screen with a frozen cursor, a pic of the bottom of the mouse, an a tablet-looking picture. I don't know anything about computers, how can I get the computer back to normal?