Laptops are typically powered by an internal rechargeable battery that is charged using an external power supply, which outputs a DC voltage typically in the range of 7.2–14.8 volts. The power supply is usually external, and connected to the laptop through a AC connector cable. It can charge the battery and power the laptop simultaneously; when the battery is fully charged, the laptop continues to run on power supplied by the external power supply. The charger adds about 400 grams (1 lb) to the overall "transport weight" of the notebook.
Power Failure
If the problem is power related (whether battery or a question of the laptop not turning on) the first step is establishing that power is getting to the laptop. This means checking that the LED on the transformer brick is lit, and if it isn't (or doesn't have an LED), that it's plugged into a good power outlet. You can check that by unplugging the transformer and simply plugging in a lamp. Some of the oldest notebook models have an internal transformer, so the line power (110 VAC in the U.S., 220 most other places) goes directly into the laptop body.
The next question is whether or not any of the little LED status lights on the laptop light up with the power plugged in. Even the oldest models usually have a power good status light. If you have positive power status and the notebook simply won't turn on, the next check is the battery. Some models of notebooks will not operate without a good battery installed, but most will, so Google up your particular model with a search like "operating without battery" and find out if your laptop will operate with a dead or missing battery. If the battery isn't an issue and the laptop still won't turn on one with the power good status light lit, it could be a switch failure, but it's more likely a power regulation or mainboard failure.
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