Do you have some kind of proof that CIC has approved the document? You can print that and show your employer. Also, contact the CIC call center and ask an agent to send you kind of proof.
Unfortunately, without an original of that document, it likely isn't possible to renew your SIN. Go to the nearest Service Canada office with any proof that the document has been approved and has been mailed, explain your situation and see if they can help.
As for your employer, I think the most important part is having the SIN number and you can explain the situation to them. They can choose to wait until you get the SIN renewed or let you start. Hope it goes well.