This thread is so I can find the solution easily when it happens again a year later and I’ve completely forgotten, to make it more likely to show up in search results for others.
On a recently rebooted Windows 10, I had opened up Discord, which loaded but was just a blank grey window. Closing it and opening it a few times did nothing, and neither did directly killing the discord process in the task manager.
What did fix it was having the window in focus and pressing Ctrl+r a few times, which apparently forces a reset of the app.