What do you see when you crank it? White vapor coming out the exhaust is fuel that won't ignite. Nothing = fuel not reaching the cylinders.
What's your weather? below freezing, and the diesel has gelled? In that case you need to warm the garage, or replace the diesel in the fuel filter with room-temperature warmed fuel. You can buy fuel additives to reduce gelling.
You can test the Thermostart. (intake manifold heater) Twist the key left 10 seconds, then feel the manifold around the thermostart module (looks like a spark plug, with a fuel hose and an electric connection) to verify it is warming the manifold.
And on most Yanmars you can crank it with the compression released, then drop the release lever to let the inertia of the flywheel assist the starter.
Tell us what you see.