I tried resetting the computer, and that fixed the issue for me. Make sure testing cheats are on (crtl+shift+c) 'testingcheats on" enter. Then in the game press shift and click the item, select "reset object (Debug)" and then if you click the computer normally you should hopefully see "Participate in Hackathon" with the little purple tent icon. I hope this helps!
Unfortunately I only think this works on PC.
Unfortunately, this doesn't work for me. I have an apartment in the same neighborhood as when the GeekCoon appears. I've found that when I get the notification "GeekCoon" is in town, I leave the neighborhood and then travel to it back from another one. That worked fine, but took me a while to figure out, cos it's not like geekcons appear every other day. Also, If I leave the neighborhood during the challenge and not before it, then the computers don't even glow as If I've participated in the challenge. This IS a game-breaking bug, but I learned my lesson and won't be paying EA any money anytime soon, that's for sure because they don't care about bugs.
