Some details are missing here. What is the $18,000 for? If you're not sure, call them up and ask. Is it for last semester, which you are still working out with them? Or it's for this semester? If you had federal student loans, why would you owe the school?
Can you find a cheaper school? In my area we have public 4 year universities that are good quality, but tuition runs less than $8k per year. Even with room & board, you can get out for less than $20k a year full time. Plus we have public 2 year colleges, where you can actually complete the first two years for half the cost, and be guaranteed a transfer to one of the 4 year schools of your choice. Good luck!