Okay, now I understand.
You sent LTC to an address generated from a bitcoin wallet (bitcoin Address)
Litecoin and bitcoin have the same address scheme so the transaction went through.
You say you can cancel it using a key, that should be the private key.
(It should start with 5, K or L)
Download Electrum-LTC from here: https://electrum-ltc.org
Install it, then you can import the private key for the bitcoin Address starting with 3 into it and access your Litecoins.