Nick,
You need a voltmeter that can measure 240 volts AC.
Step one would be to reset the main circuit breaker, even if it looks ok..
Step two is to measure the 240 volts AC coming into the main enclosure.. It is very important that you measure between the two 240 volts inputs called L1 and L2... Do not measure between L1 and ground and then L2 to ground and add the two reads together. This can result in an error.. You must measure between L1 and L2 and get about 240 Volts AC
If you do not have 240 volts, then either the main house circuit breaker is bad or the wiring has an open.
If you do have 240 volts AC, then you need to measure the output of the System transformer.. You can not measure between one output and ground, you have to measure between each pair of wires.. You can do that right at the power connector on the main card.
Tell us what specific problem you are having.. It may have nothing to do with the main power.
Do you have Pentair remotes or do you have ScreenLogic?
The more you tell us the better help we can provide.
Thanks,
Jim R.