My laptop won't switch on ,only keyboard lights

System model?

Unplug the system, disconnect the battery from the mainboard and hold the power button for 30 sec.  Remove and reinstall  the memory module(s) if they're removeable.

Reassemble and try booting again.

I did that and now it doesn't even light the keyboard keys and also it is now not charging. The battery is non removable

@Shingie does the front have any diag codes? with the system powered off, press the power button while watching the front "power and battery-status light" and see if there are any "POST" codes. The status light on the front of the laptop will blink white/amber a certain number of times based on the type of error, if any.

If you see a code (as certain count of blinking amber/white), you can lookup your code in your model's service manual, or the owner's manual depending on the model (some models have two manuals, service and owner's, while other models seem to have a consolidated single manual with owners and service information. I don't know your model so cannot say... but it should not be hard to find, figure out...

For example, if you have an "XPS 15 7590" you might do the following...

1. Web search for "xps 15 7590 service manual" and top search result should be the link to the service manual (only click a dell.com link). Note, for XPS 16, I notice this information is in the "owner's manual" ... so depending on your model, you may have to click different dell.com links.
2. Once at that dell.com link, I like to click "PDF" to load the PDF service manual, search for "amber" or "Power and battery-status light" to find the Diagnostics section that lists the various codes. 
3. The codes seem to differ by model... but the point is you will see a code blinking on the laptop you can correlate (lookup) in your model's service or owner's manual.