$16k in debt due to lapsed insurance

I was on tricare and got remicade infusions consistently for about 7 years. I turned 21 and didn’t realize I was kicked off insurance as I was waiting for school to start. I filled out my tricare form when I went in and was not notified nor was I notified when I went to the ER same night for chest pain. I learnt about it when I scheduled an appointment for the cardiologist and was put back on tricare because I started school before my next infusion.

Apparently I have a $16,000 bill sent to collections now, what am I able to do about this.