Hi Sushil,
Assuming you've exhausted all the possible software remedies (reset/wipe/restore/etc.) then there are only about two other things that can go wrong with the power button. There is the actual physical button you put your finger on and push, and under that is the electro-mechanical part consisting of a circuit board with a domed metal contact that makes the electrical connection when the button is pushed.
Either part can cause problems, but fortunately both are replaceable. Here's a guide showing you how to replace the circuit board.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/nokia-53/nokia-3-638/pulsadores-laterales-5111|Manuales / Nokia 3 / Side push buttons|new_window=true]
There's a different guide just for the physical buttons here.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/nokia-53/nokia-3-638/teclas-de-volumen-y-encendido-5112|Manuales / Nokia 3 / Volume and power keys|new_window=true]
The parts themselves are readily available; for example here's the circuit board.
[link|https://www.aliexpress.com/i/2251832696584324.html|ZONBEMA Side Key Button For Nokia 3 Volume Side Power Switch on off Button Key Flex Cable Replacment Parts - AliExpress|new_window=true]