Have you checked your fuse box?
EDIT: If you're sure your bulbs haven't blown, next thing to check is the fuse box. It's usually a small cabinet on a wall somewhere, not much bigger than a foot square. It consists of a whole lot of fuses laid out in a row. If you're living in a reasonably modern place (ie the wiring is less than 40 years old) the fuses will have switches on them. The switch gets thrown automatically if there's too much current going through the circuit. This prevents your wiring from overheating and melting.
A common problem with lighting is that one bulb blows and creates a short-circuit. Another is that an insect gets in the switch, climbs between two points, gets fried, and it's charred body creates a short circuit.
Where I come from, a switch pointing upward means power is running through it - ie, that's what you want. Once you've located your fuse-box, and can see all the switches, look for any pointing in the opposite direction to all the rest. If you can see one like this, flip it the other way and see what happens.
A man can be himself only so long as he is alone, and if he does not love solitude, he will not love freedom, for it is only when he is alone that he is really free.