Ususally the Mojang logo comes up for about 3 seconds, disappears then there is a blank screen for 1-2 seconds and the main menu comes up.

Now, the mojang logo flashes on screen and the app goes blank for about 10 seconds. Then the app crashes and I can't play.

To trigger this, (I think this is what did it) I tried to install a world from the internet. (https://hypixel.net/threads/zombie-apocalypse-minecraft-adventure-map.662/) and the app tried to import it, got to the end and crashed.