alisa- if your defense is that it was a long time ago and they should have caught it earlier that is not a defense. Based on what you’ve posted, it is entirely possible that you received government funds to which you were not entitled (i.e. money paid on your behalf to the college for a course that you didn’t end up completing). This is going to follow you around regardless of where you try to enroll in the future.
You need to get your paperwork organized- construct a timeline which shows what you paid when, which classes you registered for and which classes you completed. Someone in the bursar’s office will have their own timeline- what their records show, i.e. date of registration, date of payments, etc. Together you guys will be able to figure out if there was a posting error (money that shows up as having been paid which was misallocated, you have a similar name as another student, etc.) or if in fact, you have a balance you need to pay.
The point is not for them to prove anything. The point is for you to help them sort this out until you can wipe the slate clean.