When I try to start an arena run, I get to a screen that says “choose your hero”
However there is no hero to choose from, its just a blank teal canvas and I can’t do anything but hit the back button.
Did you watch someone play an Underground arena match?
I am returning to hearthstone after a hiatus of many years, I don’t believe I have ever spectated an underground arena match
Are you playing on PC or Mobile? Over the last week there were some issues with Arena and Battlegrounds.
Try Scan & Repair on the PC and on Mobile maybe clear the cache of the app.
The Known Issues have been updated today with this issue you described: