If you are in a union you can file a grievance and go to arbitration to get your job back. Most unionized employees already know that so I am going to assume you do not have a union. In that case, your only options are to either try to prove to the employer that you did not do what they are accusing you of, and hope for mercy, or you could file a lawsuit and fight the termination as a wrongful termination on some basis. An employer being wrong about whether or not you committed a policy violation is not a good basis for a lawsuit in most circumstances so you would need a better argument. The only way to figure out whether you have a better argument is to talk to a lawyer. Most employment lawyers will at least talk to you for free to hear what your case is about and let you know what your chances are.
Washington is an at-will employment state, meaning that an employer can fire an employee for almost any reason (provided they don't fire you because you are in a protected class). With that being said, some employers create rules and policies for themselves that may limit how they terminate their employees (through employee handbooks, employee contracts, agreements with unions, etc). I suggest you review any handbooks or policy manuals your company has to see if they violated any company policies. If you think they did you may want to consult with an employment law attorney.
Also, if you truly didn't violate the policy, you have evidence of that, and you want your job back, then there's nothing wrong with providing that evidence to your employer and seeking to be reinstated. Good luck!
Providing this general response does not create an attorney client relationship.
I agree with the two other lawyers. Unless you are union, your options are severely limited. I would check employee manuals and policies regarding employment and termination. If you think they violated one, it could be a contract of employment violation and you may want to seek out an attorney to examine the facts.