How to recover files from encrypted HP drive

Hi all,
I have an HP laptop, and I encrypted the whole drive. When I did that, I needed to store a key of some sort, so I put it on a USB stick. Its folder is called HP Drive Encryption.
Recently, my laptop died, it gives a bad smell when I try to boot it. I have warranty, so I want to send it back and have it repaired, but first, I want to recover some files from the hard drive to be sure. So I took it out of my laptop and plugged it into another pc. However, it's unreadable to that computer, which should have to do with the encryption. I don't know how to use the key on my USB and so I have no idea how to recover my files. I don't seem to be able to find similar issues elsewhere. I have a backup, but only a quite old one, and I really want to make a recent one before sending it away.
Thanks.