Try to run a "repair disk permissions" in disc utility.
- From the Go menu chose Utilities to open the utility applications folder. Find the application named Disk Utility and double click it to launch it.
- The column at the left side of the Disk Utility window lists the available disk drives with the volumes they contain indented below each. Click on the indented volume with the name of the disk you want to check. Odds are you’ll only have one choice, a volume named “Macintosh HD”, unless you partitioned the Mac’s drive or added extra drives.
- The right side of the window should already be showing you the First Aid tab, otherwise select it yourself.
- Click the Verify Disk button at the lower right corner of the window to check the volume for errors. If no problems are found then congratulations, you’re half way home! If problems are found and the Repair Disk button is enabled click it and keep your fingers crossed that Disk Utility can tidy things up for you. If that button is dimmed you are probably checking the Mac OS X startup disk which, unfortunately, can’t be repaired while you’re running from it so you’ll have to use one of the other methods that follow to attempt a fix.
- Assuming you passed the first test you can now click the Repair Disk Permissions button to find and fix problems with file ownership and access rights. It’s somewhat normal to see a few errors here especially after a software update and with printer drivers and internet plug-ins. Problems here are generally not critical but repairing permissions has been known to fix a few obscure issues from time to time.
- You’re done! Press COMMAND + Q or choose Quit Disk Utility from the Disk Utility
Also see HT1452 http://support.apple.com/kb/HT1452
I can imagine how scary this can be, but there are several things that can be done before a restore is warrented.
If you have another Mac running the same system you can copy the app from there. Also, re-installing the Combo Update may work. Also booting holding cmd-r and re-install will be an install "in place" and should not effect any of your data or other apps.
Sorry, my mistake I'd been over in the ML section for a while.
Lion is the same.
Or, you might try the 10.7.5 Combo Update
EDIT: If you had a backup this would just be a drag and drop, or a Time Machine restore.