EEOC Discrimination Disability/Retaliating

Off 5 months for work injury/Disability. EEOC sent charge of discrimination of disability and retaliation just based of the termination letter I was given. After 3 days back to work I “ voluntary resigned” and handed a check and escorted out the building while trying to file a grievance against my supervisor for harassment. I’m not sure what will happen from here I do have a Disability and did not voluntarily resign. The EEOC intake coordinator said the letter was enough evidence. And couldn’t believe someone wrote it. So they are a non profit and I do have emails requesting internally I wanted this competed and they just kept blowing me off. I’m hurt I was a volunteer there for 2 1/2 years before I became paid staff.