It was working just fine before, and then there was a big site update recently and everyone got logged out and had to re-authenticate. I am just now getting around to re-logging in on my mobile device, and I can't do it.
I've tried just "hexbear" as well as adding https:// and no matter what it just says instance not found 
Any ideas?
Liftoff isn't compatible with the current version of Hexbear, or indeed any other up-to-date Lemmy instances. The development of Liftoff has temporarily frozen because the developer has a kid now, so you'll have to wait a while and use a different app in the meantime.
Well, shit. That explains it. Thanks!