Nothing Comes On... NOTHING
First Thing the Tech Will Check IF NOTHING WORKS
First thing to check is INCOMING power. Never "think", always check. You MUST have knowledge to work with electrical. If you do not contact someone who does have.
To check your incoming power check the breaker(s). You probably have TWO. One close to the spa and one on the house main panel. Check both. Turn the switch fully OFF and then ON. Always pull breaker lever fully OFF. That resets a GFCI breaker. Turn ON. If you don't hear the breaker trip again and the spa starts up again you might have solve the issue.
If nothing, do this:
Example of an actual control box with cover open.
Turn OFF main power. Remove cover.
In our example you will see the TWO HOT power wires coming in on the lower left (circled).
This is the Main Power connection. In this example there are TWO Hots and a green ground.
On 110-120vac there will be a Black and White wire and green ground.
On 4 wire units, There will be TWO HOTS, One White Neutral, and a green ground.
If you are not qualified to measure electrical current have someone who is perform the following.
Test spas operating on 110-120vac, you will place a probe on white and one on black wire. You should have 110-120vac on your meter.
To test spas operating on 220-240 you have one of two configuration.
a. As shown, you have two "hot" wires. The wires can be red/black or the same color. As shown you test the 2 incoming terminal points. Your meter should show 220-240vac. Test hot to hot; never hot to ground. If you don't have 220-240vac then your house electrical is the problem.
b. If your spa had FOUR wires, Green, Red, Black, and WHITE, again you test Hot to Hot. You do NOT test Hot to WHITE. This will give you a faulty reading.
You meter MUST read proper incoming voltage. If not, contact an electrician. The fault is the incoming power.
CONCLUSION OF TEST
If proper voltage IS applied to spa then likely the problem is your spa. One usual possibility is enough voltage is present but not enough amperage. So your readings say its OK but not enough power is present to operate the spa. This is not a common reason.
If proper voltage IS NOT being applied to the spa then your problem is upstream. Most common its a breaker issue. Less common an issue with the wiring of the spa.