Ok I have copied the wallet dat, now do I reinstall the wallet and then copy the wallet.dat into the reinstalled wallet?
The wallet opened, but I have no coins in it now

how do I get them back? I still have old copies of my wallet.dat
Shutdown your wallet. Rename the current wallet.dat to wallet-0coins.dat ,, then copy your good wallet.dat into that directory and start up the wallet again, let us know!