Are all breakers and switches turned on?
Reach-ins, display cases, prep tables and other self-contained units* usually have only one breaker and may or may not have an on/off switch. Walk-ins may have more than one breaker plus an on/off switch.
Breakers: Walk-in freezers may have two breakers; one labeled fan and lights, the other condensing or compressor. Walk-in refrigerators normally have only one breaker for both the fans and the condensing unit.
On/Off Switches: For both types of units, these are normally located in one of two places, a.) inside the walk-in and mounted to or on the wall near the evaporator coil**, or, b.) on or near the condensing unit.*** 
Unfortunately, it's quite common to find these turned off after a delivery.

Is the thermostat set to the proper temperature?

Is anything restricting air flow to the condenser or evaporator coils?

*self-contained units are preassembled units you just plug in, like your home refrigerator.
**the evaporator coil is the cooling unit inside the walk-in that houses the fan motors.
***the condensing unit can be on top of the walk-in cooler or in a remote location like roof top, mechanical room, basement, etc.