That's a weird one. I notice in your profile you mention 3.' beta - so is this a LR2.x or 3.' beta problem?
If, with LR closed, you go find the catalog, is there a *.lrcat.lock file next to the catalog?
Is LR installed in the usual place? If you hold down Opt while starting, does it allow you to create a fresh catalog?
(Don't worry about all the questioning, just narrowing down where the problem is).