Can my company force me to voluntarily resign??

I have been really sick this last year and have been on a medical leave with the company. Technically I am out on an ADA accommodation. They had me have my doctor sign a release to work form so I can return on Monday. My doctor signed it stating that I can work 4 hours a day but my company wants me to return full time 8 hours a day. I am not abke to get the paperwork fixed before Monday and they are saying they are not allowing me anymore time off so if I don't have paperwork saying I can return to work full time they are going to consider it a voluntary resignation. I don't agree with this and said I will not voluntarily resign. They said due to staffing needs they can't accommodate part time work for me, even temporarily. I plan on clocking in on Monday regardless of the paperwork. Can they force me to resign like this or is there anything else I can do? Obviously I am trying to contact my doctor to have him update the paperwork but it's already the weekend by the time my company tells me this. What can I do to save my job??