The majority of problems like this with mobile games have to do with bad installations, or install files and other junk data being stuck in the system memory. In either case, please attempt the following:
Perform a hard reset.
Uninstall the app.
Perform another hard reset.
Re-download and install the app.
Perform a final hard reset.
Attempt to run the game.
In fact, it is generally recommended to attempt a hard reset before and after installing any large or graphically intense app, or after having any app crash or fail to launch.
If that does not resolve the issue, it is suggested that you check for updates to your device's operating system and firmware then repeat the steps above.
Additionally, devices which have been "jail broken" or have non-standard operating systems or modifications tend to encounter problems. These modifications eat into the device's system memory and processing power, reducing the assets it can focus on running the game.