By my guesstimation that demo is gone forever. The Recent Matches tab displays demoes stored on a cloud, and most probably they are simply popped/deleted from the list as it overflows - Would be costly to allow an unlimited amount of storage space per user.
Your best bet would be to contact valve. I had no luck figuring out what they system entailed through google.
EDIT: It would seem that the .dem files might be time-based stored, not just popped when > num.
Read more: How can I get a match demo that I can't download?
Might be that they do both! But if it's time-based, it could be possible if you contact valve that it is still in their systems and that they could give it to you.