November 2018 - last edited November 2018
Try running a repair as per this link here.
If it's trying to download an update and it's not moving past 0%, follow the steps here.
Lastly if you are using the streaming version of the client, one option you can do is switch to the non-streaming version as per here as it generally has less issues then the streaming version. Keep in mind this is like downloading the game again.