When you use Electrum as the interface for your Ledger hardware wallet, Electrum will ask you to create a new wallet. But this wallet alone is useless. It contains no private keys because all keys are kept on the hardware device.

OP is confused. He doesn't have a .dat file, just a regular wallet file created by Electrum. That file alone does nothing because it needs to be coupled with your Ledger device to open.

@marimanga
What happened with your seed? You needed those words when you configured your new wallet. You then had to enter them on your device as a security feature to check that you wrote them down correctly. To be able to enter them, you had to make a copy of them somewhere. Any chance to regain access to that copy?