Just learned collection agcy sued me without notice, what to do?

State of OK, debt now 6yrs past limitations, not to mention written off. Now collection agency, only the 10th one who bought debt since. I know cuz friend looking court records, come across my name. Suit was filed by lawyer in Tulsa Co Jan 2,2013. I have never received any notices, since I have been in Iowa full time since Dec 2009. I do know, however, that they have been sending mail (tho NO summons or registered mail) to my son’s address in OK. I have NEVER lived with him at that address. He moved there when I had to let my house foreclose. I had good job FT on Ticket to Work Program through SSA, cuz I’m disabled and tried to return to work. Lost that job, they didn’t pay me the $16k in severance pay. Didn’t expect them not to pay me, no, no signature. Health failed thereafter income is still SSDI. No assets of course, rent assistance. How do I handle this suit?