You don't give much info Odysseus but I'm assuming that the engine does actually turn on the starter motor? If it does and won't start, then it could be a fuel blockage in the lines or tank. Try testing whether there is fuel at the pump and whether it is entering the engine. If there is fuel in the cylinders then it may have a compression problem.
If there is no fuel then you may have a bad leak or a blockage. (I had a blocked feed once after a bee had got in the tank while I was refuelling and it blocked the feedlines!) It may even be that the fuel pump has failed and you need a new one. Maybe the pump is just about to fail and there is insufficient fuel entering the cylinders.
If the engine doesn't turn then at all then the problem is electrical such as battery or starter motor.
I can't say much more without more clues but I hope this helps.