How long did it take to start the first time? Is the car cranking rapidly or slowly? A drained or weak battery, or a starter with corrosion will not start a diesel well. Check your battery and starter. See if a jump makes the starter turn faster.
Try letting it glow for 30-40 secs (until you hear a loud click from the relay, driver side front) and see if it starts.
If those don't solve your problem, you may have bigger issues, but usually a sound diesel engine will start with adequate glow and a good battery and starter.
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K