Hi Ramesh,
You’ve done industrial level test with your hobby project. Hobby project are not meant for permanent installation or 24*7 running.
There could be many reasons and the first thing that comes to my mind is the oscillator issue. The low quality crystals may stop working properly after a long period of continuous operation. That’s the reason why the microcontrollers have internal oscilators also. If there is an option to switch between internal and external oscillator incase any of them fails, you must enable that option.