Thank you Kiara_wael, I restarted my MacBook and that seems to have done the trick and at the moment I am not getting the greyed out screen. Wish I had tried this before deleting my App on my iPhone and losing all my boosters.
I also have this problem, I am on level 17113 and cannot really progress, as I deleted the App on my phone, lost all my boosters and just keep getting the grey screen. Might be a good time for me to give up on this addictive game.