Yes you would need to do what is required as per the closure letter. That likely means having your Doctor fill out the form again as well as yourself. If you do not have the money for the Doctor to pay for the form be upfront and tell your Doctor that you will pay -it just might take a month or two.
Sun Life forms are here Forms | Workplace Benefits and Retirement Services
If your work made your condition worse and failed to accommodate then you may have a workers comp claim. A long shot, but you could make a claim if your Doctor is aware that going back to work exacerbated your illness or contributed to your decline.
Insurance is a legal contract. Your obligation is to prove you can not work and to take all steps to get better.
If they don’t pay you then ultimately you can sue.
You should immediately send a request for your complete claim file and the Master policy for your group plan -send this request to privacyofficer@sunlife.com
You should also inform your employer that your Doctor has taken you off work for a month and you will be sending in your claim forms. Request to review the Master policy from your employer so you know what is required. Do this in writing.
Read the post in this forum Proving to Insurance company I was unfit to work
With your application include the documents I listed if possible. You need to explain why you can’t do your job and have your Doctors support.
For loss of money-use the food bank, welfare is also for those who are disabled-there are ways to qualify for funds.
If you have a mortgage, loan-call the bank as often there are options to skip a payment once a year.
I am not aware of your illness-make sure that your Doctor is making referrals if needed.