How do I commit suicide?

Having looked through pretty much all options and controls in the game, I have not ever seen an option to commit suicide. So unless you are playing a hero who has splash damage, you cannot kill yourself. If you are not playing a hero with splash damage, your best bet is just to quit the match so your team can get someone able to do stuff.

You may also want to report the issue to Blizzard so they can maybe figure out what is wrong and fix the bug.

If you are playing any character with explosives (Junkrat, Pharah, Zarya, Soldier 76), you can aim your explosives at your own feet to blow yourself up. In normal circumstances, you could also jump off a cliff or into a hole to kill yourself (obviously not possible in your scenario).

Other than that there is no way to kill yourself. You would either have to wait until the round is over, or until you get disconnected/quit the game.

You may want to post this bug on the forums as well, in case it's something that Blizzard can fix.

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NOTE: As of the patch on March 21, 2017, Junkrat can no longer damage himself with his own bombs.