Can my employer demote me due to being disabled for the past 6 months?

The answer is "maybe" or "it depends."

The law prohibits discriminating in employment against someone because he or she is disabled. However, the obligation on employers is only to make "reasonable accomodations," which are accomodations in duties, equipment, facilities, etc. which are not too costly or disruptive. An employer is not required to pay someone to do a job which he or she simply  cannot do. So if the demotion is not punishment, harassment, discrimination, etc. for being disabled, but is because you could not do your old job and are being given a job which you can do, that might be legal.

Also, while the employer may not discriminate due to disability, being disabled does not prevent the employer from taking action based on other grounds. For example, if you had unexcused absences or performance problems, your employer may be able to demote you for those reasons, even if you were disabled at the time.

It is therefore a very fact- or context-sensitive issue: in many situations, demoting a disabled worker is illegal, but there are some circumstances where it may be permissible. For a more definitive answer about your precise situation, you should consult in detail with an employment law attorney. Good luck.