If you have the physical sticker but the key is "already in use," you can sometimes provide a photo to Steam Support to reclaim the license. If you'd like, I can help you:
While later Valve titles and Steam-integrated versions shifted to the 25-digit alphanumeric format standard for Steam product keys, the classic 1998 "WonID" versions of Half-Life, Opposing Force, and Blue Shift relied on the shorter 13-digit sequence found on the back of the jewel case [2, 3]. Why You Might Be Looking for 25 Digits half life 1 cd key 25 digits