There is no limit to taking the wallet offline. The blockchain, which is shared all user, contains all the information on who owns the bitcoins. Your private key identifies the ownership.
Now the reason why you may see a balance of 0 is because the official windows client doesn't seem to store the wallet inside the application folder, but in your OS user profile. It may still be reading the wrong wallet.
If you see your old transactions, but a balance of 0 - then you should worry.