Launcher won't open

Put the summary of the bug you're having here

What I expected to happen was...:
The launcher opened

What actually happened was...:
There was a thing saying 'downloading runtime' with a bar showing how far it was getting but when it was finished it just closed. If I open it again it pops up again. I cannot get to the launcher. This started when the new launcher came.

Steps to Reproduce:
Open the launcher