It isn't with the other breakers.

Not outside near the heat pump?

While there will be a disconnect at the exterior unit, can be either fused, or circuit breaker, or even non-fused, there certainly will be one in a panelboard, someplace.

Are you sure you know the location of all your fuse or circuit breaker panels?

It may be labeled air conditioning.

It may not be labeled at all.

If this is an apartment, there may be a breaker in a landlord panel, if the owner furnishes heat and AC. Slim chance but still a possibility.

There is one labeled upstairs a/c, is that it?

My guess is yes.