This morning my N97 mini was on 2 bars battery life and I dropped it onto a mains USB charger for about 10 mins.
One hour later, wondering why I was hearing lots of email notifications from my iPhone but none from the 97, I checked only to find the thing completely off, so flat so it would not turn on, and took a few seconds to come on even once mains was connected.
i think something like this had been discussed before, if you have a 97 or mini, do not let the batteries run flat, else its likely that a hard reset will be the only way to resolve it. even if you leave the phone plugged in the whole night once its flat, it doesnt seem to push any juice into the battery and it will not come up when you try to switch it on
yours seems a little different though, that it has 2 bars and it didnt charge..