BlockChain.Info Incorrect Balance
As long as you keep your private/public keypair, you aren't in danger of actually losing the funds. 

The problem is most likely due to "missing inputs" for your current wallet. In order to spend bitcoins, your wallet must reference the inputs that gave your wallet the funds in the first place. The function of the blockchain is to keep track of these inputs from creation to their current wallet. Without the full blockchain, qt will not be able to reference inputs for spending, and thus won't show such inputs as existing in your wallet.

Download the blockchain. If you can't/won't sweep you private key into a new wallet that has access to the full blockchain. This will also serve to reset the oldest input in your new wallet.