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.

When you install Electrum-LTC, open it, when it asks what kind of wallet you want to create select "import Litecoin Address or private keys" and click next
Then paste your private key into the field that appears and select next
Then the address starting with 3 should show, and you can now spend your Litecoin.