Put your battery charger on, make sure you have a good strong battery at this temp and you might want to put a small heater under your motor, but at this point it is your option.

Next while the battery charges, Check your glow plugs and make sure they are all good.
If they are all good make sure the battery is good and strong and cycle your plugs 3 times for 20 sec. each time, use your watch.
If the above doesn't work you will have to get a small heater under the motor and get it as warm as possible and repeat the above.

I had the same problem you had, one day it started fine got cold the next day and would not start, I checked my glow plugs and I had 2 that were good, I changed them all.
You should get some anti gel for your fuel if you are below freezing for long.
Good Luck! Jim