What can I do if my water cannot be turned off?
The main cause of valves becoming stuck is when they are opened fully and left there. The correct way is to open them fully and then close them half a turn or so. 

This enables a little 'wiggle' room for the valve when it slowly becomes stuck again over the years. A valve stuck hard against the stops is very difficult to persuade to move again. 

On stuck valves I always use a penetrating lube the day before, WD40 or such like. Try a little gentle tapping around the valve with a timber stick at the same time as trying to 'rock' the valve back and forth. 

Some valve designs have a nut which can be slightly loosened to relieve pressure on the seal/washer etc. Be prepared for some small weeping/leaking when the valve finally moves.

Use extra leverage like wrenches at your peril, you might get away with it or it could go horribly wrong...