Heat pump will not switch from heat to ac... help

Model number and brand of unit?

Bad reversing valve
Bad reversing valve coil
Check for 24 volts at the coil If present check for coil continuity/OHM if defective replace.

Valve itself will have to be professionally replaced since it is brazed in place.

If you replaced the thermostat lately that could also be a problem if not set up or wired correctly.

Make sure wires going to the outside unit are OK. Dogs/animals chew on them.