Why won’t my Mac book pro turn on

check the followiong

Is it plugged in?
Are any lights coming on?
have you checked the charger is working on another machine?
Is the socket turned on?
Can you remove the battery and then plug it in and turn it on?

I mean, there are so many reasons. Those are just a few.

Yup plugged in.
Only lights turn on is charger light.
There's no latches to disengage the battery
Don't have another max device to test charger

Socket?

the plug socket but if the light is on on the charger it's not that.

Connect the power adapter to the Mac. On the MacBook / Pro's keyboard, hold down the Shift+Control+Option keys and the Power button at the same time. Release all keys and the power button at the same time


http://osxdaily.com/2010/03/24/when-and-how-to-reset-your-mac-system-management-controller-smc/

Read the article
Did the keyboard combo.
Didn't power on.

The article said the socket light might change colors but it didn't.

What happened with this laptop before it stopped working?  Was it working before?  Did you run down the battery?  Is this an old Mac you picked up from somewhere?  Please describe the events and circumstances before you got to this point.

Apple devices will not power up if the battery is below a certain  level.  If the battery is fully drained, you'd have to charge it for at least half an hour before you can turn it on.  While you may still be able to reset the SMC, it won't come back on if the battery is dead.

Not mine but trying to fix for friend, BUT they said it was working fine when just writing a letter. Came back 2 days later battery dead.

Tried to reboot after charging for hours and sometimes it would boot up sometimes not.

Now it's not powering on at all.

Did she spill something on it?  Did you see the battery level when it was on?

How old is it?  What model is it?  https://everymac.com/ultimate-mac-lookup/

When I press the button on the side all 8 lights turn green and remain lit for a few seconds.
 It's a MacBook pro9

https://everymac.com/systems/apple/macbook_pro/specs/macbook-pro-core-i5-2.5-13-mid-2012-unibody-usb3-specs.html

Serial#: C1mlk3cndty3

It has the keyboard cover as well as the top/bottom plastic covers to prevent spills. Older friend that has desktop discipline. So no spills I know of.

Then I suspect the battery is dead or dying or the charging circuit isn't working, or both.

How do I test that

You can't if it doesn't turn on.  You'd need someone that can open up the laptop and test the battery or the charging circuit.  If it's partially powering on, maybe try the SMC reset again.  It's a model from mid 2012, so the battery is 6 years old and probably has been recharged enough cycles.  It's likely time to get a new laptop.