I agree with Ryman: unless you have it in writing that financial aid would cover all expenses, you may be out of luck. There are a lot of reasons why financial aid might not have gone completely through, and it's not just limited to Phoenix Online; it happens at brick-and-mortar universities too.

My sister used to work in the financial aid office at an online university (not this particular one). She said the reps would lie to applicants all the time about financial aid, and then her office would be forced to take the eventual angry call. She complained to the higher-ups many times, but to no avail.

Anyways, good luck getting this straightened out.