I have a Trezor and made a wallet online on mytrezor.com. But what happens when the site goes offline for some reason? I don't have a private key as far as i know. Anyone an idea?
You did write down the seed for your Trezor and keep in a safe place right? If your Trezor is lost you need that seed phrase to recover. If myTrezor.com is down briefly for maintenance you can always fire up a new installation of Electrum, select new wallet and hardware wallet, then Trezor and manage your bitcoin in Electrum.
If you have lost the seed to your Trezor you should buy a new one, set it up and send your bitcoin from the old Trezor to new one. See the
Trezor user manual. Most Trezor users own more than one just in case of loss or breakage.