Aircon should never just need regassing, it's a sealed system. Sometimes the seal's dry out if left for a REALLY long time with no use.
Normally, the cause for it not turning on is the LPS (low pressure switch), so if there's no gas in the system, it doesn't even try.
The normal cause for it not working is a split condenser. Easy to replace yourself.
It's also a great idea to head to quickfit, where they do a "not working, no charge" type thing, they basically find the fault for you, you fix, and then get them to re-gas after.
If there's no fault they just fill it up and you're away.