Water pressure in upstairs sink lost after turning off main line to fix shower
When you shut off your main (or any valve really but the main is the big one) and turn it back on you can cause debris or sediment that was trapped somewhere in your plumbing to dislodge. 

It seems to me that some debris might have gotten trapped in that faucet's aerator. Remove it and see if the pressure seems fine. 

If the pressure seems fine with the aerator removed, clean the aerator.

If there is still low pressure try to close and open the shutoff valve for that faucet (with the aerator still removed) to try to dislodge anything that might have gotten stuck in that valve but chances are it's the aerator.