Try some basics such as Repairing Disk Permissions in Disk Utility. If you are not aware of DU, it's in Applications - Utilities - Disk Utilities. If that doesn't help the reset both the iMac SMC and PRAM reset at least 2x each. If that doesn't work then post back.
ATTENTION EVERYONE WITH THIS ISSUE
Last week I encountered this, i booted up my computer, I was not able to use anything on my computer besides programs on the dock. I could not click on restore desktop/dont restore desktop. I was out of luck because I had all my important docs on my desktop and POOF! they were gone so I paniced. however, with some steps, I was able to recover my data. I tried created another admin user to access docs, that did not work. I tried everything you can think of, everything I can try with 6+ hours of research on this bug. first things work, before you become a hero on the CLI, start up your system holding CMD+R, then backup whatever data you can, I used an external harddrive. Afterwards, I was left with one choice, and that was entering "sudo rm -rf ~/ .Trash " Be very careful, this is a very powerful tool you type in command prompt. If you are like me, you could not simply open command prompt using spotlight, so you need to force open spotlight by using CMD+Space, then type terminal, then the code above^^^. After you do so and you access the docs you recovered, you will realize you cannot open the files that were previously on your desktop. However, I was bummed but I later simply made a copy of the Desktop folder containing the files, moved it to a exterior harddrive, reformatted my harddrive back to exFat,(I had to format my harddrive for the recovery) transferred the data, then opened the files using a windows computer. Hope this is of any help everyone, I really struggledd on this bug with the Lion OS so I hope this will solve any problems you may have.
FINDER DID NOT WORK
ONLY DOCK WAS ACCESSIBLE
ALL FILES ON DESKTOP DISSAPEARED
SAME PROBLEM EVEN AFTER REINSTALLING THE OS
SIMPLY PUT: USE THE SUDO CODE, WHICH CLEANS YOUR HOME FOLDER, AFTER YOU BACKUP YOUR DATA
GOOD LUCK. THIS IS A LOT OF INFORMATION, DO NOT BASH ON ME IF YOU DO SOMETHING WRONG, I TAKE NO RESPONSIBILITY FOR ANYONES ACTIONS.