I also updated the firmware but the balance was still zero!
I've done multiple firmware updates without problem, but you are alays warned to make sure you have a backup of the mnemonic prior to upgrading. Are seeing the same wallet addresses on the new firmware ? If you have no passphrase you should see the same addresses, and if you have entered the same passphrase again you should see the same HD tree. Do you have any old transactions in from phone wallet to Trezor accounts you can check ?
I think that this one is already resolved. (wrong passphrase was used)
But in general, if you plug in your Trezor and you don't see your transaction history and you see balance zero, there is nothing to worry about. The website is down/slow/whatever, but your coins are almost certainly secure. The only situation to worry about is when you see a transaction history with some new transaction you did not approve. (I've never saw someone report this)