In alt.home.repair, on Wed, 30 Jan 2019 09:42:38 -0800 (PST), Davej
wrote:

No water in one of the upstairs bathrooms this morning. Thinking of
cutting a hole in the wall to try to figure out where the coldest
area is. In the basement I can see where the pipes go up. It is an
interior wall but quite close to the exterior wall. Any suggestions?

You think your pipe is frozen?

How close is quite close to an exterior wall?

How cold is it in your house?

You seem to think the frozen spot is upstairs. Maybe it's downstairs.
How many holes will you make?

Both the hot and cold water won't run?

If you have other sinks, and I can tell you do, why not wait until
things warm up. Maybe you'll have water then, with no effort on your
part and no hole.