My iMac screen has a text box on it that I cannot close or remove

Press the key combination command-F5 to deactivate VoiceOver. If that doesn't work, see below.

For OS X 10.7: From the menu bar, select

 ▹ System Preferences ▹ Universal Access ▹ Seeing ▹ VoiceOver: Off

For OS X 10.8 or later: Select

 ▹ System Preferences ▹ Accessibility ▹ VoiceOver ▹ Enable VoiceOver: Off

There is a few things that you can do to try and fix this.

1. Try completely turning off the computer, and leave it off for say 30 min. I'm not sure if this will work though as you have tried restarting it already.
2. Try deactivating voice over (command and F5) to see if thats the problem
3. Try to put it into safe mode, as this may fix the issue.

This is a little hard to try to understand as i don't have a picture of it. Posting one would behoove you in this situation. I do have a few questions for you though.

• What have you installed recently?
• What's in your Accounts login items?

Hope either I helped or someone else can help you fix this problem. Please consider posting a picture to help out with this issue.

Post a screnshot of it. Also boot into Safe Mode, Mac OS X: Starting up in Safe Mode, and see if it continues to appear. Is there any title in the top frame of the text box? Any text in it? What system are you running? What apps do you have set to launch at boot in the System/User & Groups/Login Items for your account?