Often, by the time Yongnuo flashes shut down due to heat, it's already at the point where they're permanently damaged. It sounds like that has happened with your YN685.

This just comes with the territory of buying cheap flashes. They're usually very good until they quit working but they tend to quit working much sooner than their higher priced counterparts.

I've personally had the same thing happen to me with a YN-568EX II in the past. In the case of that YN-568EX II, it will power on and give one full power flash (regardless of the power level setting) and then not do anything for about 30 minutes or so.

I use Yongnuo flashes extensively and have had pretty good luck with them. My primary speedlight right now is a YN685. Most of the time I use it for fairly low-powered fill flash which tends to not stress a flash as much as frequent full or near-full powered bursts would.

But I always have a Canon 430EX II with me 'just in case'.

The time that a YN-568EX II fried itself (I had been using HSS for fill in daylight - which also demands a lot of power in short time periods), having the 430EX II with me allowed me to continue shooting at an event that started around noon and lasted almost until midnight.