forced to resign

Ive worked in the for this medical company for eigteen plus years, never had a bad evaluation, took on the role as a manager for 1 1/2 years recieved bad evaluation from manager and director based on he said. All evidence was based on as my manager stated "I need to be approachable" and staff say this, Manager would never sit staff down to get to the bottom of this so one day I decide to call staff and manager together. Staff was lieing and my manager was a witness but she would never speak up for whats right. She took this information to the director and twisted the entire event. More to the story, I was told by the Director that the organization had no other need for my service and that she know that I've been working for over 20 years here at the organization. How can evil people get away with falsely accusing people.I was never shown any letters that was written towards me. Only what he/she said to the managers. My Director told me that if I resign I will receive a service pay but I can no longer be an employee.