I have tried that already, closing down the computer, and restarting-no joy at all. I think i am going to have to remove the old wallet and reinstall. Do you know if I can manually input the original address into the new wallet?
Only if you know the private key - which I don't think bitcoin-qt (the client application) tells you. It's most likely not the wallet's fault: be sure to back up the wallet. If you delete/lose the wallet file, any money sent to the addresses in the wallet
will be lost. There are programs that can recover the private key from your wallet, but it's pretty complicated.