With a game as large as Gems of War, sometimes your phone or tablet might start running out of memory chunks big enough to load everything. This is called fragmentation, and it's easy to fix by restarting your device.
Try powering down your device completely; this is often done by pressing and holding the power button on the top or side of the device.
If you restart your device, wait a few minutes before powering it back on. It will clean out the memory in your device and make sure that there is plenty of space to start loading up all your apps (including Gems of War) once again.