Power button not working

@soumenmandal here is the Vostro 3500 Service Manual Vostro 15 3500 Check page 15. The power button is actually part of a replaceable module. Page 63 of the manual will tell you how to replace the power button. Looks like replacement modules are available at plenty of online sources. I suggest that you find a vendor close to your geographic region.

While you work on your laptop, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy.