No cold water in bathtub faucet

Bathtub cold water suddenly quit. Hot water faucet still fine. Removed and examined cartridge. All OK. I also had wife stand by as I did a quick turn on and off at the home shut off valve. No water pressure! All other cold water faucets work. There must be somthing plugging the bathtub cold water line. What is best approach to clean out? This is my mother-in-law's home and quite old plumbing.

Could be anything, but if this is a three handle (hot/cold/diverter) setup, it could be as simple as changing the washer. When changing the washer, sometimes you need to change the stem, and may want to replace the valve seat, but is might just need a washer.

I presume the entire stem/washer/seat/cartridge was out when you had the wife on standby with your test. I don't know where Lovington is and whether a frozen water line is within the realm of possibility.

[edtit]OK, since the OP updated his profile, it is not likely to be a frozen line.

A stuck balancing spool in a Moen "MoenTrol" or Delta "ScaldGuard", (or Danza, Pegasus, etc), could act that way since it does NOT come out with the cartridge.