Based upon the facts you have provided, you very well may not have been competent to understand what you were signing and you may be able to argue that your were coerced into signing this document, and that you do not agree with any facts therein. There may even be some common law tort claims that can be brought against your employer. The best thing to do is to set up a meeting with an employment or labor law attorney and explain everything that happened. That lawyer will then be able to determine if you have a cause of action, or even just should write a demand letter requesting you be reinstated and the document be stricken from your personnel file.
Any information provided here is for information purposes only and is not to be construed as legal advice or intended to create an attorney client relationship. Attorney Michael Goldstein is licensed in Massachusetts only and is associated with counsel in RI, MD and DC.