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.