Are you using a very old Ledger device? In that case, I know they have since changed the derivation path. One thing to try: on the MyCrypto app, input your seed phrase, then on the next screen, change the derivation path to m/44'/60'/0' (instead of the usual m/44'/60'/0'/0) and see if your account shows up. This is done by scrolling to the bottom of the derivation path options and selecting "Custom".