Good evening and thank you for entrusting me to assist you. I am very sorry to hear about your employer's decision to let you go.
Unfortunately, I must tell you that the facts you describe would not typically give rise to any legal claims. The reason boils down to the fact that employment is "at will" absent an express agreement to the contrary. At will employment can be terminated for any reason not amounting to discrimination on the basis of a legally protected trait (race, religion, gender, etc.) or retaliation for engaging in certain forms of legally protected conduct (filing a wage claim, taking FMLA leave, etc.). It doesn't matter whether the basis for termination is fair, reasonable or even TRUE.
Thus, you can be terminated despite a great review, despite the lack of warnings, and despite any complaints about your competency being completely without merit. Employment is essentially a voluntary relationship that can be terminated by either party at any time.
As for severance, your employer has no legal obligation to pay anything beyond your actual earned wages. You can certainly try to negotiate severance pay, but an employee's main leverage in negotiating severance is the threat of a lawsuit, and based on these facts, your employer doesn't really have any legal exposure. So, you just don't have much leverage. Also, by pushing too hard to risk burning bridges, which your employer is apparently quite willing to extend to you right now. So, I would really be careful in pushing too hard for severance, and in general it would be a poor idea to get a lawyer involved as the lawyer won't be able to do much for you and will most likely just create animosity between you and your employer.
I realize that this is not what you were hoping to hear, and I am truly sorry that the law does not provide more protection. But you deserve and honest and direct answer to your question, which is what I am trying to provide.
I hope that you find this information helpful. Please do not hesitate to let me know if you have any questions or concerns regarding the above and I will be more than happy to assist you further.
If you do not require any further assistance, please be so kind as to provide a positive rating of my service so that I may receive credit for assisting you. Very best wishes moving forward.