VPN
If you are using a VPN, stop your VPN or refer to the article Genymotion Desktop and VPN.
Hardware virtualisation feature is disabled
Make sure that the Intel VT-X, AMD-V or SVM virtualization feature is enabled in your BIOS. Please refer to "Your CPU is incompatible with virtualization technologies" error for more details.
Windows 8/10
Please refer to "Unable to start the virtual device" (Windows 10)
If none of the above worked for you, try to re-install VirtualBox and Genymotion (See How to uninstall Genymotion Desktop completely (purge)?)