My MacBook pro won't turn on

Try the following in order as needed:

1 - try to boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

2 - try to boot into the Recovery volume (boot with the Command + R keys held down  - Intel Macs) or  (How to Boot an M1 Mac into Recovery Mode). If that works then select Disk Utility and repaid all volumes that DU sees.

3 - if still can't boot normally repeat #2 but this time reinstall the system.

If your Mac doesn't turn on

Writing an effective Apple Support Communities question

Is this a New Issue ? If Yes, what exactly changed on this computer ?

Is this an Old Issue ? If Yes, what actions has the user taken to fix this issue ?

After all,  we do not what to suggest possible solutions for what has already been attempted by the User 

Understand, we are all working Remotely from all around the world. 

Therefore, we do not have the On-Hands experience the User ( you ) has with this computer.

Reliable , accurate, details and context will really help to pinpoint a possible solution.

As Phillips has offered, we need to know what MacBook Pro you have as different models operate differently (Intel or Apple silicon)? Also what do you mean "will not turn on?" Does it try to boot and where does it fail? Did it work before? Did you install any software or make any changes and then it stopped working?

I assume that you've tried booting it with the charger plugged in?

To get an intelligent response, give us a hint as to what's going on.

Help us out here, what does "Yes just downloaded new OS. Won't power up at all. Won't even charge" mean?

Are you saying that you downloaded the OS with the computer that now won't power up? Did you do anything between downloading the new OS and trying to power up the computer?

Does "Won't power up at all" mean that you don't get the boot logo or what? Does it try to boot and fail?

How do you know it doesn't charge? If it won't boot then you shouldn't be able to see the battery status which is only available after the machine boots? Also did you note what type of computer you're talking about?

You will get better answers if you spend a minute and describe exactly what you've done and the results. "Won't power up," could be a thousand things if we don't understand exactly what happened or didn't happen.