Start. Opponent selected. White screen. Hang. Loose.
This happens occasionally (recently it is becoming more likely) and you can't see why it does sometimes and not others.
Start game. An opponent is selected. It goes to a white screen. The white screen can be there from a fraction of a second to indefinitely. I have left it up to about 20 seconds.
If you force kill, then start again, the game does not give you the option of rejoining the existing game. There is no message to say you have lost but you know you have because points have been deducted.
It sounds like you may need some more troubleshooting advice, please submit at: https://magicspellslingers-support.wizards.com/hc/en-us/requests/new and select 'Technical Troubleshooting'.