Like most Hitachi systems this range of units have 2 core 5v dc interconnecting communication cable between outdoor & indoor units.
But as the 5v dc fluctuates its almost impossible to measure with a multi meter.

The indoor unit will have its own 220 -240v power supply & if that is off then you wont have 12v dc to the remote controller & the display will be blank.

If the indoor unit did have mains power to the circuit board then it would report an 03 communication alarm if it cannot get any communication reply from the outdoor unit on the 5v dc interconnecting cable.

The flashing LED 1 & 2 at the outdoor unit might be reporting communication error with indoor unit.

BTW
if the indoor unit has mains 1ph power then check the transformer.
If it has failed then the RC will be blank as it works on 12v dc from the indoor board.