Run the repair utility, it means your install is corrupt. when you run the tool it will likely say wow has suffered significant corruption and needs to be reinstalled.
this seems to be a side effect of multiboxing as its happened on two hard drives now for me but only since ive started boxing.

once you get a patched up and working copy i suggest you make a copy of it and just use it to make new copies from as they get corrupt.
im going to get a new hard drive soon so ill get to put my mboxing vs harddrive shitting out theory to the test.