Adobe digital editions old account - don't know logins

Hi 

I'm just trying to read an ebook from my library, I'm not tech savvy or real good at finniky tech stuff.

Apparently I have an old acocunt with Dymocks, which I don't know the log in or password for, and I can't deauthorise that one, to log in with my library account and read the ebooks.

Adobe chat said they don't support this app thingo, and sent me here.

Please help.