Either someone stole all his coins as a result of him storing his wallet.dat on a cloud service (also known as "someone else's computer"), or more likely the wallet was never his to begin with, and he has either stolen a wallet.dat file or been duped in to buying an empty wallet.dat file from a scammer. Given that OP seems to have moved on and forgotten about it pretty much immediately, I would be very surprised if those 10 BTC were ever his to begin with.