The ADA requires "reasonable accommodations". The law is clear on this.
So if your employer is not providing any accommodations? They are in violation of the ADA
It is important to understand that the ADA does not require that an employer change your job description, or assign you to a different position...only that they make "reasonable accommodations" for your disability.
That said, what you describe seems the opposite...it seems they are not providing any accommodation at all. Again, that is a clear violation of the ADA.
The first step would be to go to HR and complain. But what you describe, it sounds like you have tried that already. If HR will not help you, then your next step is the EEOC.
The EEOC can step in and help resolve this via their complaint process.
You can make the complaint to the local EEOC filed office (you can find the one closest to you here http://www.eeoc.gov/field/index.cfm
Once you file the complaint that will start the process where the EEOC will investigate and make recommendations to the employer. If the employer continues to refuse to comply with the law, after you make your complaint, you can file suit under the ADA and sue for the damages you suffer.
So you have rights. Specifically the right to a reasonable accommodation for your disability
I understand your fear of retaliation...it may be that happens. But at the end of the day, if you want to enforce your rights, and if HR will not help you, the next step is to file a complaint with the ADA
Please let me know if you have more questions...happy to assist if I can