This is a plumbing question that needs to be addressed quickly. The most likely explanation for a high water bill is a leak somewhere within your home's water system. Look carefully at all faucets for leaks and check toilets to see if they are running even when not in use. Check all appliances for leaks, particularly the water heater. Finally, make a careful assessment throughout your house, looking for damp spots on the walls, floors and ceilings that would indicate leaking pipes inside the walls. If you are unable to locate the problem and fix it, call a plumber.