Water Pump Belt
If the washer won't drain the pump belt might have broken or might have worn out. Check the belt and replace if it shows signs of excessive wear.
Drain Pump
If the washer won't drain water the drain pump might be defective. It's also common for a small sock or other article of clothing to get caught in the drain pump or in the drain hose. Check both for an obstruction before replacing the pump.
Other Causes and Conditions
Clogged Pump or Hose
If the washer won't drain there might be a clogged pump or hose. It's common for small socks or other small clothing items to get into the water drain system and clog the hose leading to the pump or the pump itself. Remove the hoses from the pump in order to remove the article of clothing. Keep in mind that any water in the washer will pour out of the hoses when removed.