Why Aren't Any of My String Light Bulbs Lighting Up?

Every single light bulb in my string light set does not turn on.

It is most likely an issue with incoming power to the string light strand.  Here are some things to check.

1. Check the breaker box in your electrical panel to see if the socket is receiving electricity.  Plug in a different device to see if it has power.
   
   
2. Try resetting the GFI receptacle for your outdoor outlet.
   
   
3. Check to be sure the bulbs are not loose in their sockets. There is a copper tab at the bottom of each bulb socket. Remove the string lights from power and make sure the copper tab is sticking up at a 40 degree angle. Check each socket and then screw in each bulb nice and tight. Power up the string lights to see If that fixes the issue.

Here's a video with more tips