You have a chat account on a chat server. In the network, there are 3 servers:
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
When you're desperate and don't mind running an userscript for a couple of ... days you could use this chat scraper script.. When you have it installed and visit an chat userprofile, like mine on MSE Chat, it will start processing all rooms and list the number of messages per room. During processing this is what you'll see:
When a room is complete it will show the number of messages posted in that room. If the room has no messages from you it will be removed from the list. The script currently visits all rooms, frozen and deleted ones. Set
nohide:true (click to see where) to change that behavior.
The script makes calls every 4 seconds to prevent being throttled by the SE network. I can't guarantee though that you won't hit an temporary IP-block, so use with care or make it call even less often.