You can't import the .key file directly... especially if you put a password on it.
You need to export the keys WITHOUT a password, then you need to open the .key file in a text editor... and copy/paste just the private keys into Electrum. They start with a "5", a "K" or an "L"...
NOTE: doing this increases the risk of exposure of your private keys... I suggest doing this OFFLINE and doing what you can to ensure the machine is free of malware etc.