How to repair my power adapter

Yes. More info indeed.

If - as I assume - it's a 5v mobile phone adaptor, and the previously mentioned items are OK and the output of the adaptor reads zero volts, then repairing the circuitry inside becomes a problem assuming once more it is a regulated or switching supply unit. Non-regulated supplies basically consist only of a few components (transformer, capacitor, resistors, led indicating light) and the faulty component could sometimes readily be visually spotted or measured and then replaced.

A bit of a futile exercise because these adaptors are cheap to buy and opening them is tricky because they are hermetically sealed and have to be carefully prised apart. However it can be done - except if it is an iPhone adapter which is almost impossible to open and if opened the components are so minute and tightly packed that testing/repair is out of the question.

Try a different wall outlet and check the cable for any cuts or loose connectors. If it still won’t power on, the adapter is likely dead — most modern chargers aren’t repairable, so replacing it is usually the only safe fix.