Fired from a job due to a disability.

While working for a large cable company, you discover that you experience great pain while performing a part of the job function (after employed for aprox. 6 moths.). You fail a training course due to the pain from your knees. Your employer tells you that you must see a doctor before you can retest. The doctor tells you that your pre permanently disabled from the position that you are currently doing due to a knee/joint disability.
You go back to your employer to give them the information and you are asked to work in the dispatching department to accommodate you for the disability. You accept the position but are called on the telephone a few hours later by your supervisor and told that you are not to report to work and you are to “just hang in there until further notice”. They fire you saying that you were “hiding a disability and unwilling to comply with companies request. I had never stated that I had a disability but they told me that I had to put on a report that I had experienced pain in my knees in the past. But everybody had pain at one point or another but I have never experienced pain like this before. I had never been diagnosed in the past until I worked for this employer. Can they fire me because of this disability? I was told that I should do something about it but I was told by the American Disability department that it could take thousands of dollars to prove my case and it would not be worth it. Is somebody hiding something? - Is this your question? Add additional information