I don't think 'unavailable' is the problem. Cllunlu want's to turn the power to the PIC off from the regulator under software control but still be able to turn it back on again under software control. When the regulator is turned off, the PIC no longer has a power source so it can't run any software at all.
There are solutions but some experimentation may be needed. My first thought is to provide an alternative VDD supply to the PIC, one that stays on all the time but put the PIC into sleep mode after shutting the main regulator down. That would reduce current consumption to minimal levels (a few uA) but still allow it to be brought out of sleep by interrupts or timers. Anything else the regulator feeds would be completely shut off, I'm asuming Cllunlu isn't using a 3A regulator for the PIC alone!
Brian.