There is an immediate undo option that appears in the UI header when a session/window/tab is deleted.
The user data file can be found according to the instructions in the following link:
Saved sessions are stored in the "SavedSessions" table. Deleted sessions are flagged with a "deleted" field value of 'true'. So you can do something like this to select deleted sessions:
select * from savedsessions where deleted='true'
Then you'll need to copy the JSON from the "windows" field of each session and parse it to extract the windows/tabs from the session. An online JSON viewer might help (eg,
http://json.parser.online.fr/)
NOTE: soft-deleted sessions are purged when the browser is restarted.
I hope that helps.
Hans