Sorry to hear about your health problems, but this really is why all travellers should take out travel insurance.

It sounds strange that you would go to another airline and presumably book one-way travel (perhaps it was reward travel?). It's standard practice that tickets are cancelled once any sector is missed, and your actions show that you were resigned to having lost the outbound without even contacting the airline. I really don't think KLM will re-instate the ticket.