Main account hijacked

Noticed a few days ago that I wasn't logged in and my credentials didn't work. When I entered into the "forgot my password" option and put my email, it sent me an email for THIS account, which is NOT my main one. I am now seeing my actual account posting comments on subs I have never interacted with. My email doesn't show ANY notification about changing passwords or emails, so, what happened here and how can I ask for help to get my account back?