First, as previously mentioned, get the petrol system checked. I would imagine the fuel pump may be main suspect...
As for LPG, it is entirely possible the tank is either empty, or it "thinks" it's empty!
When I got my car, some damn foo had been fannying around with it and the level float was not only squint (which meant it was reading wrong) but they had over tightened it until it cracked the filler elbow so each time I filled it, the vapour would leak out of the filler hose (only the hose mind you, the tank was safe as the shut off valve was fine).
I digress, my point being that my tank only appeared to hold 9 pounds worth of gas and would run out, yet it still held a further 30 odd quids worth. So, it could be the filler valve is telling the gauge either wrong info or no info, either way, the ECU wont open the tank shut off valve to allow the gas to flow. Or it could even just be one of the shut off valves broken or no feed. My system has one at the tank and one at the vapouriser.
To check there is gas at the vapouriser, crack off the nut of the main feed (from the tank) and see if there is a hiss & a whiff of gas (then close it up again for obvious reasons) at least that will show fuel is present.
Next you need to check for a spark at the plugs. Easiest way is have a spare plug and pull off a lead from one on the engine, plug the spare plug into it then lay it on the engine to earth it and get a friend to spin the engine over.
If a spark is present, then you have number two out of the 4 requirements (3rd being compression & 4th being exhaust)
I can't imagine you have no compression unless there was a catastrophic failure which I'm sure you would have heard...
Now I said exhaust because it's not the 1st time some arse has shoved a tattie or two up there
Most LPG systems (like mine) start on petrol then temperature rise means the vapouriser gets it's heat from the coolant to allow it to heat the L (liquid) part of the equation and turn it into a gas which is then injected and burned like petrol.
So your system is missing something it previously had before it ran out! Could be a sticky valve...(the shut off valves I mentioned before) but to be honest, it you are not mechanically minded and have no previous LPG experience, I would get hold of someone who has. But for pity's sake, DO NOT let dangerous Dave (made up name) near it, or anyone with no common sense or mechanical knowledge.
So petrol... These motors are known for the pump to fail and while a bit of a dick to replace, not beyond the scope of a competent mechanic.
Does the car have a standard petrol tank or a teeny wee thingy like mine had?
And either a boot mounted LPG tank or spare wheel well mounted tank?
So much to be getting on with, but until we know the specifics, it's going to be hard to trace either fault
