I logged at someone's account using my e-mail. Idk what to do.

So, around 1 year ago I created an e-mail. Today I wanted to use this e-mail to create an alt account on tiktok, but apparently someone already used this e-mail and I'm on someone's account now. It seems like the account wasn't online for like 2+ years. Idk what to do now, cuz on one side, I should own the account cuz I'm the owner of the e-mail, I created the e-mail, created the password, ext., but on the other, the account exists for more than 2 years and I didn't create it. idk what to do now.