First, close bitcoin-qt and stop bitcoind if it's running. If there is a wallet.dat in ~/.bitcoin, back it up and then replace it with the one from your USB drive. The restart bitcoin-qt (or bitcoind) with the --rescan flag, and give it a few hours (depending on your system) to rescan the blockchain for transactions in that wallet. Example: "bitcoin-qt --rescan". If that returns something along the lines of "bitcoin-qt: command not found", make sure you're in the same directory you compiled bitcoin in and run "./bitcoin-qt --rescan", or make sure it is installed properly and try again without the "./".