If they did supply air before then here are some things to check. Also, is the air flowing out from the other supply registers as strong as before?
1. Filters are clean? This is unlikely due to the sudden onset of the problem but check them anyway.
2. Nothing has damaged the duct work... no recent construction or, if flex duct, no animals chewing into the ducting?
3. Is the blower motor working properly. Check the blower motor for any smell like burning rubber.
4. Nothing was dropped down the floor register blocking the duct? Remove the grill and look as far as you can with a flashlight. Kids love to play with registers.