My HUD has completely disappeared! What happened to it?

After picking my hero in the pre-game lobby, I typically Alt-Tab out of the game to browse the web for a short time until the match starts. This time, when I Alt-Tabbed back into the game, my HUD completely disappeared! I could still control my character but I could not see my health, ult charge, ammo reserves, aiming reticle, nothing! In hero select for the next round, I could not see any heroes to choose from, so I had to stay with the hero I picked from the last round.

When I restarted Overwatch, my HUD was back on.

Why did my HUD disappear? How can I get it back if this happens again?

So it turns out I "fat-fingered" a key combination on accident while trying to Alt-Tab. Overwatch (and apparently most other Blizzard games) have Toggle HUD hotkey which is set to AltX by default. I probably hit that key combination on accident while switching programs.

This feature is nice if you're trying to capture a screenshot of a scenic vista, interesting landmark, or particularly gruesome death. However you can disable the hotkey by pressing Esc and clicking Options > Controls > Toggle HUD and unbinding the hotkey for the command.