It is not the type of thing you can sue them for. However, I do suggest that you follow up with your financial aid office to obtain an explanation about the reason the money was returned. Did you stop attending school, or are you no longer enrolled at the school? Was this a federal loan that was only for tuition, and not for additional expenses? There are several types of federal aid, and if this amount was only supposed to be applied to tuition, they might not be allowed, by law, to disburse it to you. However, and this is more likely, they may have inadvertently returned it, because they were not aware that you needed the additional amounts.
Speak to your financial aid officers. Lawsuits don't solve these matters, and they only delay the arrival of the money (and only a portion of the money after legal fees and expenses).
This does not constitute legal advice or the engagement of my services as an attorney.
Ask the school to request that the overage be returned to you.
I am an Attorney-at-Law, licensed to practice law only in the state of California. Unless we have both signed a formal retainer agreement, you are not my client, and my discussion of issues does not constitute legal advice. Opinions expressed herein are those of the author, and do not necessarily represent the opinions of those who hold other opinions.