The following steps also help with troubleshooting launching the game.
Assure you are running as Administrator
Set H1Z1.exe and launchpad.exe to run as administrator (Right click the program or shortcut and choose the option Run as Administrator)
If you are on Windows 8, try changing the compatibility mode of H1Z1 from 8.1 to 8.0
Right click on H1Z1.exe and choose Properties
Select the Compatibility Tab
Check the box for Run this program in compatibilty mode for:
Make the desired change
Use Steam to verify the integrity of the game cache, you can find intructions through Steam HERE.
Logout and back into Steam.
Validate the game assets using the launcher
Run H1Z1
Login, but do not click Play
Click on the gear icon near the bottom left
Choose Validate Game Assets
Click Validate
Wait
Try completely uninstalling and reinstalling H1Z1 (you will want to reboot as well.) This step is near the bottom for a reason, don't do it unless you have to.
Try logging in from another computer. (We've seen several cases where players were able to successfully link by logging into Steam and the game client from another location.)