If I received a summons due to a defaulted student loan and I have 20 days to respond, what do I do?

First, respond--file an answer that denies liability and expresses no opinion about other factors (such as the identify of the lender; you can say "Lacks information so as to form a belief and leaves plaintiff to its proofs.") Only admit the most obvious--e.g. your name, if that is correct. If you don't file an answer, you lose by default; and anything you admit in the answer can and will be used against you.

If this was a government backed or government insured loan, you cannot discharge the debt in bankruptcy, so that option is foreclosed you. So longer term, once you answer (which at least buys you time), you need to consider what to do--do you try to settle for part of the debt or work out a payment plan? Do you feel you have good grounds to contest the debt, or at least part of it (did they credit you all payments you made?) and therefore will fight it? Do they have you dead to rights but you have no money, in which case, you have no effective recourse but also little at stake? Etc.