One faucet doesn't work when it's really cold
As others state, fix it before the pipe bursts, and you have expensive water damage, possibly making the house unlivable.

Temporarily, you might try putting insulation over the outside of the wall where the pipe runs. You could try following the path of the pipe with a stud finder, though if it's plastic pipe, it might not show. Then make place bats of insulation outside long that path, and cover them with a wood frame to prevent that insulation getting damaged.