My site is closed to new registrations, I don't want it to be!

Hi guys, my server ran out of space and having freed up a load, my hosting Co. tell me all is fine now

However, although existing user can login and out, the site won’t accept any new registrations for some reason. Only error it gives is ‘ERROR: Couldn’t register you… please contact the webmaster !’

Webhosts guess is that the tables need repairing, which they tried to do but made no difference

Anyone got any clues what’s causes this kind of issue and how to fix it please?

Thanks!

Pete