I understand how you feel. I was in the same situation last semester. They wouldn't let me back in and also had to write a letter and attach documents. Basically, they just want to know how serious you are about continuing your education. Explain to them in detail about your situation in the letter and make it very clear that you will be doing better from then on. Attach any pay stubs, receipts or anything that you can show them to prove to them that you were in a financial situation. If you don't really have anything to show then, your appeal letter needs to really detailed and convincing. That's pretty much what I did and I did get back in. Hope this helps.