No HW on Second Floor

Good Afternoon, I live in a 2 story home and discovered this morning that I have HW on all levels of my 2 story home (basement bath, main level kitchen and PR), but not the 2 baths on the second floor. There is no apparent leak and no water flow when the faucets are turned to hot water. Any obvious things I should do or look for? I very nervous about having a pipe eventually burst (especially during the night).

Look for air leaks, missing/displaced insulation.

This assumes you're in a cold area.............

check your hot water heater and see if you can hear water is flowing.

I'm assuming you've got freezing temps outside... do the pipes run in an outside wall or are they exposed to potentially freezing temps in any way? Do you have any way to warm them and keep them from freezing?