If the story is as you say it was, then immediately:
1. Check with the department's HR/admin to make sure what was said in the letter you received wasn't a mistake.
2. If its correct, ask for a copy of any and all disciplinary paperwork, file a FOIA request if they refuse to give it to you.
3. get a lawyer, and fight it.
4. Document any and all conversations/phone calls/letters/emails (to include method of communication, date/time, duration, who was present, what was discussed) you receive from anyone employed by, or associated with the department going forward. Keep it all in a file. If you are in a one party state, record any conversations. Any electronic communications like email, print out a hard copy to be safe...
If you were fired, and worse yet, fired for/with pending disciplinary actions against you, then you have a slim chance of ever getting another job in LE. Fight it now, or decide you never want to work in LE again, and move on to a new career.
The liability a department takes on when hiring someone they know has had "issues" in the past, is too great for most to accept. Even with the shortage of cops these days, a department 99.9% of the time, will pass on a candidate if they know they are a lawsuit waiting to happen.