I really need more information here, but hopefully the following covers manty of the possible scenarios.
Do you hear a motor spinning during the time the drum should be turning?
If you hear a motor spinning inside the machine then there is a good chance that the drive belt has simply come off. You may be able to just take the lid off michine (My article http://www.fixya.com/support/t1543617-door_seal
covers how to do this), hook the belt over the small spindle on the motor and then put it on the big wheel on the back of the drum by placing it over one side of the wheel and spinning the drum to draw it around the rest of the wheel.
Now, if this is the case, you are going to have to ask yourself why it came off. Sometimes this will happen if you had a very heavy load in the machine and the spin cycle threw the machine all over the place, so what was the machine doing on the last occasion it DID work?. Sometimes, the belt comes off because the drum bearings are worn out and there is movement in the drum. To check for this, open the front door of the machine, put your fingers on the drum on the inside and try to lift the front of the drum upwards a little. Bear in mind that there is an outer drum, with the drum that spins inside it and what you are feeling for is the inner drum moving inside the outer one, this will be obvious and "Clonky" if it is worn out. You are not looking to see of the whole drum assembly rocks smoothly inside the chassis, It is supposed to do this. If your bearings are worn out and this has caused the belt to come off, it may well be accompanied by a small, but worsening leak from the back of the machine as the water slowly leaks more and more through the bearing. I'm aftaid that if this is the case, then, depending on the age of the machine it may be time to scrap it. Certainly a bearing replacement is possible as a DIY job, but it is a real pain and not for the faint hearted.
So then, on to the scenario where you CANNOT hear a motor running during the wash cycle when the drum should be turning. This is probably the motor faulting out. Once again, a few checks: does it NEVER run, or will it still spin the washing? Your motor will have up to 4 electrical windings inside it, One for the standard turning motion when there is water in the drum during a wash and up to 3 more that kick in at various stages of the spin as it gets faster and faster. If it still spins, but will not wash, then it is almost certainly a burned out winding in the motor and you have to replace the whole motor. This will cost you (as a rule of thumb) about 25-40% of the cost of the machine when you bought it.
If the belt is on but the motor does nothing at all on any cycle, there might be a loose connection on the back of the motor (A plastic connector block with around 6 wires clips into the back of the motor, check it is firmly in place). Or, the brushes in the motor may be worn out. You can get separate replaceable brushes for most motors. If you can replace an altenator on your car, you can replace a motor on your washing machine, the principle is exactly the same. If you get to the point of taking the motor out, give it a good physical inspection, looking for burn evidence on the wires of the winding, water drip damage AND have a good sniff at it. If you smell that horrible burned wire and plastic smell, you know that the motor is going in the bin.