If you can't open Gems of War and you're running Windows XP or lower, this is because Gems of War no longer supports this version of operating system.
The game is coded using a software development tool called Unity. The version of Unity that the game is made in ceased support for the Computer Operating System Windows XP as of July 2017. We've tried to keep Gems of War running since then, but with the release of Update 3.5 (Hero Classes/Kingdoms rework) it seems that some Windows XP players can no longer access the game.
Why is this happening?
We wish Gems of War could be accessible to as many players as possible, but this is a problem that is out of our hands. Unity cannot run on an Operating System that is no longer supported by Micrsoft, and we can't run without Unity! Unfortunately, to release the latest update and continue adding new content to gems of War altogether we needed to switch to this newer version. For more information, see this post from Unity.
What can I do?
If you wish to continue playing Gems of War, the game still supports all computers which have drivers that are newer than 2010 and have DirectX 10 installed. You can check your version of Direct X here. Unfortunately, most computers with Windows XP installed are not compatible with DirectX 10.
Gems of War accounts can also be linked to your mobile (iOS or Android) devices. For more information see this article to link your account, or contact Support with "Account Issue" selected and we can help you set up your game on a new device.