Test the glow plugs. They should have a low resistance (0-50 ohms typical). Test the voltage at the glow plugs - you should see close to battery voltage (12v) at the glow plugs for 15-20 seconds.
If you just want to get it started, first plug in the block heater (if you have one) to get it good and warm, then put a jumper between the battery and the glow plugs for about 15 seconds - then crank.
