For stub areas, a default route is not propogated by default. You must explicetely tell the ABR to send the default route into the NSSA area.

router ospf 1
area nssa default-information-originate

The above output will generate NSSA Type-2 external default route (LSA type-7).
The metric type for the default route above can be changed to a Type 1 with the metric command.

router ospf 1 area
nssa default-information-originate metric-type 1

If the area is set to NSSA totally stuby area. An Inter-area default route will be created on the ABR and sent to the NSSA totally stubby area.

router ospf 1 area
nssa default-information-originate no-summary