How do I get the water pressure back?
Closing the main valve could have dislodged some rust / scale that is now stuck in the cold water valve. Since your hot is running ok turn the water off again and pull the valve apart.
You may have to slightly turn the water back on to fully flush the line out. Reassemble and see if you have full flow again. 
Did you purge the air using the cold valves? this is when the gunk gets pushed into the valves.