Suggestions:

1. Check glow plugs. Leave the door open, turn the key. If the dome light dims, some are glowing. If there is no dim, none are working.

2. Check the glow plugs with a multimeter. Lots of threads on how.

3. If you can crank enough to start without proper glow plugs, your battery is probably OK for now. Go back to #1 and #2 above...

4. Do your power locks not work on unlock AND lock? Listen for the vacuum pump in the trunk. If it's running, you may have a door lock vacuum leak.

Let us know.

Oh, did I suggest that you check the glow plugs?