Help With Mac Os X Grey Screen

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I bought an 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', and hold 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?

Press and hold the power button for 5 seconds or so. Your iMac should shut off.
Press and release that power button again. Should start normally.

The graphic showing a mouse usually means that you don't have a mouse.
Plug in a USB mouse, if you have one.
Is the keyboard an Apple-brand keyboard? There will be an Apple symbol of the bottom.
Is the keyboard wireless? Check the batteries.
If both the mouse and keyboard are wireless, best choice is to use a USB mouse and keyboard, as you don't know about the wireless (although that SHOULD work, too)

Normal startup using the Command-R:
Hold those keys immediately after you press and release the power button.
You should see an Apple icon on the screen, then a progress indicator, which will either be a spinning wheel, or a progress bar.
You can release the keys when you see a progress indicator.
You should then see a menu screen, with a variety of choices. One will be terminal - and you can continue with the steps that you saw in the video to change, or reset the password.

Another great answer. Thank you!