How do you shut off the water if there are no valves under the bathroom sink?

If there are no valves under the bathroom sink, there are three other possibilities:

1. There will be a valve on the line between your water heater and the sink.
2. There will be a valve on the main line that comes into your house. If you have a basement, check along the wall on the side that is nearest to the street, as this is where the line comes in.
3. There will also be a valve on the water meter, which is found at your property line near the street. The access will always have a steel cover, which is sometimes as large as a manhole.

In any case, all that it takes is a good pair of pliers to turn off the valve.