Frozen iMac cursor despite keyboard functionality

iMac starts up but cursor remains frozen in upper left corner. I can't get to About this Mac, and have several operating, so I can't give all the information I should. I can use the keyboard: example: I clicked on print and the print window opened . Hit enter and the window printed but I can't do anything with the cursor.

[Re-Titled by Moderator]

Original Title: Frozen iMac cursor