petrol or electric? if it is petrol then have u got fuel in it, oil in it, are the blades clear of debris, is the engine wearing out, is the grass too long, are u flooding the engine with too much fuel?
maybe a few more details might help the mechanical minded out with your question.
Could be the breather hole in the filler cap is blocked. This will quickly create enough vaccum in the fuel tank to stop the petrol flow. Examine the cap inside and out as there is usually an anti-spill device inside it.