On the right hand side of your screen is the Leaderboard (past 7 days). At the bottom, click on "All time top users".
This will display the top 100 users. You can then click on the user's profile.
So, if the user merely left, then you can still see everything that they submitted. If they left by deleting their account then this method doesn't apply.
Another method that may or may not work, especially if you're looking for someone not in the top 100 users, is to simply Google the user name.