Sounds like you're trying to repair the startup disk, which it won't let you do.
If you have a single partition computer and have no other options, reboot the computer and hold down Command-S. That will boot in single-user mode with some pretty weird looking text messages.
Eventually you'll be left at a command prompt where you'll type <b>fsck</b> which will run a disk repair tool. You may need to run it more than once until it reports that it found no errors.
Another nice thing to know (or suspect--it's just a rumor so far as I know) is that OS X does some self-repair on startup anyway.
-Rob