Hey there,
Most cases are that your opponent has quit. (Or your connection seems to have some problems)
Here are some few signs that your opponent has quit. (only for when the timer has stopped at 00)
1. If your opponent has been overheated after your turn, then he hasn't auto-shutdown.. (You have to wait a few seconds) and if your enemy hasn't auto-cool down within 5-6 seconds, he quits.
2. If you hover your mouse over your mech, you can see "Taunt" then click it. And if your mech haven't taunted your enemy, he quits.

(This one is when its your opponent's turn.)
Well, that's only my reasons I know when the enemy has quit when the timer got stuck on 00.
Hope this helps.
