What "everything" have you tried? If you don't say then you will just get people suggesting all of the things you have done already.
Is the phone software modified in any way? Had anything happened, or was there any odd behaviour, before this began?
In the absence of further information the obvious things to try would be factory reset or reflashing the phone. But if, as you just posted, you cannot get it to power on at all then neither of those is possible.
If the phone won't power up there is nothing you can do apart from get it repaired (which won't be as easy at the moment). The one hope for a simple fix would be if the problem is that the battery has died: in that case a new battery would solve everything with no need for resets, reflashes or anything else. And from what you say it is possible that that is the problem - I can't say more than "possible", remote diagnosis on the basis of limited information is always tricky. For all I know it might be something terminal like a motherboard failure - the battery possibility is just the one thing that might fit the symptoms which you can do something about yourself.
One thing you could try is just leaving it charging for many hours and then see whether it will boot. Note that if the battery is completely dead the phone won't start even if plugged in: the phone cannot run directly off power supplied by the charger, it always runs off the battery (plugging it in only charges the battery, not powers the phone), so if the battery can't be charged (for whatever reason) the phone cannot start up.
