If the freezer is not running at all, check the fuse or circuit breaker. Freezers require a great deal of electricity. Something else running in the vicinity at the same time can trigger a safety stop.
If the freezer is not cooling as well as it should, listen to hear if the compressor motor is humming or making a steady noise as it should be.
If the compressor is working, the thermostat may need replacing.
The freezer also may not cool properly when the evaporator coil freezes over due to dust and dirt. This requires unplugging the freezer for a manual defrosting. If you want to clean the coil, you'll need to open a panel at the back, which should be diagrammed in your owner's manual.
Ice build-up inside the freezer indicates a malfunctioning automatic defroster.
If the freezer runs continuously, the thermostat may be defective, the level of refrigerant may be low, or the freezer may be too close to a wall. This leaves the condenser coil unable to radiate heat away from the appliance.
If the freezer is noisy, it may be simply that the appliance is not level, or much worse, that the compressor may be failing.