Ummm.... my clutch grindeth [View all]

I have a '78 MGB with a custom high-performance engine (about 30,000 miles ago) and at that point I put in an overdrive transmission. It's worked perfectly. I recently stored it in my garage for the winter, though on a warm(er) I day I try to start it up. Suddenly it grinds like hell trying to go into reverse (never actually goes), and it's impossible to get it into any other gear (although those don't grind, they just don't work).

On the few times I've lost the clutch on other cars in the past it's been gradual process, but this seems different. Could there be something else going on?

Thanks

- Tab