You have a chat account on a chat server. In the network there are 3 servers:
- Stack Overflow Chat
- for all users of Stack Overflow
- Stack Exchange Chat
- for all users of all other sites in the SE network
- Meta Stack Exchange Chat
- for user of Meta Stack Exchange
Each chat server has their own collection of user accounts, you don't necessarily have an account on all three. To find rooms you've participated in, visit all three servers (or at least verify you picked the right one).
From the topbar you can open the site navigator and then find a link to chat for your current community. Clicking it brings you on a chat server that belongs to the site you're on.
In the chat topbar you'll find your username. Clicking it brings you to your chat profile of the specific chat server. If you're on the General tab you'll find on the right of the page two subtabs: rooms you're currently in and rooms you're frequently in.
On the left you see the rooms you are an owner in.
If you don't find any rooms on that page, you can check for recent messages and if that is also without success you're last hope is search