^Hawking asks this question in his book and provides multiple hypotheses as answers. Ill summarize for everybodys pleasure.(The answer to the original question)
i)"What happens in space-time must be a consistent solution of the laws of physics".
So history is consistent and may not be altered by time travelers. So you can not go back in time unless history has already recorded that you already had arrived in the past.
"moreover when you did go back(in time), you wouldn't be able to change recorded history. That means you wouldn't have free will to do what you wanted".
ii)"..when time travelers go back to the past, they enter alternative histories which differ from recorded history"
This merges with the idea that our universe is a 4 dimensional brane drifting in an 11 dimensional sea, along with an infinite amount of other branes.
According to this the infinite number of branes provide that all possible histories have been recorded in one brane or another. So time travel would be a jump from one brane to another.
A time machine would be a vehicle that can create a worm hole on demand to another point in space time, within our universe or another. However the energy required, as mentioned above by Faris is incomprehensible. Just to help people understand, a worm hole(or the Einstein-Rosen bridge)is a temporary bridge between two black holes, which exists for an extremely short period of time. So first you need the energy to create two black holes & then more energy to keep the bridge alive long enough for the vehicle to go through. Hawking says it maybe be possible for advance civilizations to keep the Einstein-Rosen bridge(wormhole) open long enough for a spaceship to pass through. He then later says that he would bet against it.
About Diwanas question about what time means. The temporal dimension is like any other dimension, but one through which humans are constantly traveling and we perceive this as time. Since our biological existence is dependent on this inertia of traveling through time and seeingthings as past or future, any time travel would require a bubble of space-fabric in which humans could encapsulate them selves to allow for our biological functions to continue. This bubble would have to be ripped from one part of the mulit-verse, transported through two black holes through the Einstein_rosen bridge and then re attached to the target destination. Without this we would not emerge at the other side as ourselves but our molecules would emanate from the other side of the black hole.
To top it all off, Hawking also says that, after the event horizon, time starts to flow within the black hole at a right angle from the flow of time as we know it.
). So what does that say? does that mean that we never got around to creating the time machine? or maybe this era is so boring, that no future time traveller wants to visit?