This is about logging in on another StackExchange site, but I think the problem applies to any SE site, including this one.
Long time ago I created an account on math.stackexchange, but I haven't been there for a while. I used to login with my OpenID account, but that option seems to have been dropped. When I login with my Google account I create a new account. They suggest I can merge a new account with an existing one, but then I have to be able to login to the existing account in the first place. Looks like I'm stymied. How do I get in again?