You must shut down the Bitcoin client and confirm it is not running before making a backup of your wallet. If you do not do this, it is likely your backup will not work correctly. This is a good way to lose many coins.
In addition, if the Bitcoin client does not find a wallet.dat file in the data directory when it starts, it will silently create a new one. Again, you cannot restore a backup over the newly created one with the bitcoin client running.
I would first be checking the 'recycle bin' if this is windows, to see if you can restore the wallet as it was when you pressed 'delete', or use other undelete/file recovery utilities at your disposal.
You should completely shut down your bitcoin client (or even restart your computer), and then see if it can be restored and check it's operation. If there is nothing to be found from deleted files, then place the wallet backup into the bitcoin directory and see if it will run. Do you have error messages when bitcoin runs?
If the wallet.dat file will not allow bitcoin to run, but bitcoin will start up and create a new wallet.dat if there is no wallet.dat in its data directory, then you need to attempt data recovery on the wallet.dat file, either using a repair utility to fix a corrupted wallet database, or an import/export tool to attempt offline export of the private keys out of the wallet. Do not directly repair your USB backup; keep the backup file untouched, as it now appears to be your only copy and you don't want to mess it up further.