As an at-will employee, you can quit at any time for any reason, you can be fired for any reason, a rotten reason, or no reason, except unlawful discrimination. If you do not have an employment contract or union to represent you, your recourse is limited. Discrimination based on age, gender, race, religious beliefs etc.... may give rise to a different answer.
You might find my Legal Guide helpful "Workplace Discrimination: A Basis for Wrongful Termination Claims"
If you have a discrimination theory about retaliatory discharge or hostile workplace conditions you'll need a lawyer. Religious belief, age, gender, race etc. all are discriminatory reasons for which you cannot be legally fired and upon which you might base a wrongful termination suit.
You might find my Legal Guide helpful "How to Choose a Lawyer for you.”
You might find my Legal Guide helpful "What Do I Tell My Lawyer"?
No one can know what the record is in the case because online we cannot find out any details. Check with a lawyer in your locale to discuss more of the details.
Good luck to you.
God bless. Best of luck to you.
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You didn't state why you were in rehab or why you were fired. Was the rehab from a workplace injury? Were you fired because you were still in rehab? These are important questions to have answered. You should talk to an attorney as soon as possible, especially if the rehab did come from a workplace injury, as the statute of limitations on filing a claim in comp is two years from the date of accident.