Almost as bad as running out of oil is running low on water – also known as engine coolant.
The reason for this is that the water in your radiator circulates round your engine to keep it cool. No water = rapid overheating and possible engine damage.
Checking your water is easy, but you must do it when the engine is cold:
Lift the bonnet of your car and secure it in the raised position.
Locate the coolant reservoir and check that the water level is between the min and max markings. If it has fallen below the minimum, top it up with tap water as required.
Make sure you tighten the cap on the coolant tank properly after topping up the water.
(The water will probably look pink – this is the anti-freeze. You don’t normally need to add anti-freeze between services.)