as wrote before, i suggested personally to invest in btc and to use electrum because really trustable and safe... so i really feel in fault if he looses 2500 thanks to my suggestion.
don't worry, you didn't lose his bitcoin. It is still there in the address mentioned
it's simply technical problem showing the balance in your wallet.
in addition to saving your seed,
you can extract individually that address' private keys just for double safety measureand do what
TryNinja suggested and also check just in case your wallet is offline mode
edit: my mistake... I was thinking desktop electrum.
is turning on/offline mode is desktop electrum feature only too?