Go to nwa.com and call up the reservation with the original confirmation number. It should still come up, although none of the flight segments will be listed. Click on the ticket number and then scroll down to see the list of segments. Each should have "Refunded" next to it. That at least shows the refund got into the system correctly.

Sinceit's been weeks and you haven't yet received either the credit or at least an email saying the refund was approved, there is probably a glitch somewhere in the system. Next step is to call the refund department (not the regular number). Hopefully they will tell you everything is in process. But they may tell you the refund was rejected. Be prepared to prove you actually did request the refund the next day. You did make a printout when it said "Print a copy for your records" didn't you? Did you get an email acknowledgement after you submitted the refund request?

Overall I've found the refunds coming a bit slower in recent months but it shouldn't take this long.