The cold water in the sink and toilet tank in our basement bathroom comes out either as a trickle or not at all. The cold water works fine in all the other bathrooms (2), the kitchen and laundry room. The toilet water connection is turned on all the way, as is the sink. The hot water in the sink comes out perfectly. There is another water connection in the same bathroom which works fine (apparently the last owners were going to put in a shower at some point). Do you know what could cause this and; if so, is there a simple fix or should I be calling a plumber? Thank you.

Sounds like some crud got in your system. Shut the water off to the lavatory and toilet. Open up the faucet and check the inlet ports on the cartridge, now place a pan over the open valve body and turn the cold water on for a moment to flush out the supply. Unscrew the three screws on top of the ball cock and pull off the float ball and washer assembly. Place a pan over it and turn the water back on. You should have a full stream from both. If not click on back. Good luck, Tom