This happened to my thumb drive that I like to carry on my keychain.
I used Velcro strips to affix it to my key fob. Do you have something else on your keychain that you could attach the key to with Velcro? If not, maybe you could add something flat to your keychain to stick the Velcro to.
The adhesive on the Velcro is VERY tacky and the thumb drive has never even thought about coming off the key fob.