Well, got my first collection letter

A bit of back-story: Went to school pursuing veterinary medicine, graduated with a BS in Biology, did some post-bacc work as well. Worked in the industry for close to 17 years (started working when I was 12), lots of experience, decent grades. Applied for 4 years and couldn't get in. Meanwhile, managed to rack up $45K~ in loans that blew up to $79K due to interest under Sallie Mae (2004-2012). Private loans are around $52,000 with interest rates between 10-13.5%. I purposely stopped paying November 2017 due to, oh, I don't know, wanting to buy groceries? Pay my rent? Pay for my car to get to and from my job? I've never been able to make above $30K a year, even with my degree.

Anyways... Got a letter from Financial Asset Management Systems today in the mail, stating that I owe them $45K due to my defaulted private student loans. Obviously, never been in a position to be able to afford to pay them anything beyond the Sallie Mae "Pay $150 so we can put them in forbearance another 3 months" and even that was a stretch. I guess my question is, do I write and ask them to validate the debt? What's the likelihood of them having all of the signed documents from Sallie Mae turned Navient? Or do I just continue to block all calls and just say fuck it?