Welcome to Sailnet!
We can be a bit more helpful if you answer a couple of questions:
1) What kind and model of engine is it?
2) Does it have a glow plugs?
3) Does the engine crank as fast as it did when it used to start?
4) Have you done anything to the engine recently?
1987 CS 36 Merlin "Kyrie"
"They drove a dump truck full of money up to my house. I'm not made of stone!" -Krusty the Clown