My most recent employer is using reprimands in my file for a period that I was on Fmla to prevent me from getting work. How can I stop this?

I worked for the same school district in the same capacity for 15 years. I had to use FMlA due to surgery and complications associated with it. During the first week and month of my return to work I was reprimanded three times for job actions I could not perform during my FMLA. At the end of that month I had another injury requiring emergency transport and treatment at a hospital. I took retirement immediately because I was afraid to return to work. I was in physical therapy through the following month with workman's comp. I applied to substitute to subsidize my income last month. Three weeks later I received a rather vague letter from the empemployers loyer stating that other applicant's were more qualified (highly unlikely). I called the employer to clarify the response. The denial for work was based on review of my file which is compromised with the reprimands for actions I could not properly perform during my FMLA. This 15 year compromised file prevents viable future work