Once it's all used up, who knows what will happen. Nuclear war to control the last coal deposits?
We're still using fossil fuels because it's cheaper to employ them as a power source and for transportation than the alternatives. When fossil fuels actually begin to run out, the cost of using them will go up, when that cost exceeds the alternatives we will switch over.
As far as coal is concerned, there is a shit load of coal in existence that is currently not being extracted owing to the high cost of simply getting at it, but it is there. Total coal reserves for America are estimated to be as high as three trillion metric tonnes, but over half of that is in north-western Alaska, very hard to get at.