Can an employer legally force you to resign? its a little more complex read for more info?

I work for a major corporation in tinley park, il and i was out on a disability claim and my employer asks for medical documentation. Then they send you two letters saying if you don't respond we will take it as your resignation, is this legal in the state of illinois? I sent in the medical documentation and... show more I work for a major corporation in tinley park, il and i was out on a disability claim and my employer asks for medical documentation. Then they send you two letters saying if you don't respond we will take it as your resignation, is this legal in the state of illinois? I sent in the medical documentation and the sent me a letter saying that i resigned and to return my keycard but i thought you had to officially had to resign in the form of a letter or something in writing which i never did. Help please
Update: P.s I did send in documentation in a timley manner and the said "we will accept your resignation" not a were firing you and I thought you had to submit a formal letter of resignation which I never did. I've been a loyal employee of 5 years and I want my job back.
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