We have heard from Several Customers that have issues with DC40 collars that won't turn on. Unfortunately most of the time these collars need to be sent back to Garmin to be replaced.
A Couple things you can check before Calling Garmin, would be to see if the Red light turns on ,or if the Blue light turns on when you connect it to a charger. If you can not get either Light to turn on it will need to be sent in to Garmin for Replacement.
Garmin's Support number is 1-800-800-1020 For more information about contacting Garmin Warranty or Repair department please visit their Website at www.garmin.com.
Here is a Short Video showing the problem with a DC40 collar.