Avoid opening/removing cover at all costs
If this doesn't work you may have a dead breaker or wiring issue. If either of these are the case you will need an electrician/service tech to check the hot tub.
Check the control to see if the heater icon is lit up or has an indicator that it is "on"
If the heater indicator light is on the computer thinks it is heating the tub and will keep the pump that pushes water through the heater running constantly. This typically means the relay to the pump is shot and the board/pack needs replacing. The good thing is that the pump will keep running which should prevent the water from freezing until you can get the board/pack replaced. DO NOT turn the power off to the hot tub.
The computer typically has an issue in this case and would need to be replaced, as in the instance above as long as the pump involved in heating is constantly running you should have time to wait for a tech to come over.
This is caused by either a defective component or a faulty GFCI (Less common of the 2)
The most common cause of this is a defective heater element. If you are mechanically inclined you can turn the breaker off and try to disconnect the leads to the heater element and then turn the breaker back on and if it doesn't trip that was the cause. Leave the tub on as the pump will keep running in an attempt to heat the water which will keep it from freezing.
If the breaker still trips after disconnecting the heater element it could be the breaker or main computer. If this is happening make sure to keep the cover closed and do not open for any reason.
The main reason this would happen is from a defective component that comes on after a delay. Typically this would be caused by the pump or ozonator (or one of the other components). If you are mechanically inclined you can do some troubleshooting. Turn the breaker off and disconnect all the components from the main computer. Turn the breaker back on and see if it trips again. If it does it is either the computer, the breaker or the electrical connection and an electrician or service technician should be contacted.
If it does not trip plug in the components one at a time turning the breaker on after each one is plugged in. If the breaker trips after connecting a component that is the issue and turn the breaker back off and disconnect it and replace it.
Most likely there has already been freeze damage that has caused plumbing to crack/break and is the reason why the water has escaped. If this is the case turn off the breaker (should be off anyways but make sure) and contact a service technician to come over asap. There is no immediate rush (that night for example) as damage has already been done and there isn't a lot that can be done immediately to mitigate the damage.
If the pump (the one that pushes water through the heater) is making noise/humming it is most likely seized. Turn off the breaker and unplug the pump (it will most likely trip the breaker if you don't). If there is a secondary pump keep running it to keep the water in the tub moving to prevent it from freezing. Replace the pump as soon as possible and if your hot tub has T valves on either side of the pump you won't have to drain the tub to change out the pump.