Not a big mistake.
Export the private key and restore the wallet using BCH wallet client. I think Electrum has a BCH software.
Electrum does not have a BCH software. It has a fork that was modified for BCH but that is not from the same author as Electrum.
Bitcoin Cash is a fork of Bitcoin and certain address types are valid for both. It cannot be reversed. Electron Cash is a fork of Electrum. You can export that specific private key and use any wallet that supports Bitcoin Cash to get your funds.