My iphone isnt turning on?!

Press Sleep/Wake and Home buttons and hold them for at least 10 seconds.

If this does not help, take a look these Apple Support articles:

If your iPhone, iPad, or iPod touch doesn't respond or doesn't turn on - Apple Support

Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support.

As a final step, try the recovery mode. Turn your device off, then plug it to your computer with the home button pressed. Keep the Home button pressed until you see the iTunes logo on the screen of your iPhone. After that on your computer you should see the iTunes window saying your iPhone needs to be restored to factory settings. Click on the restore button.

More info here: https://support.apple.com/en-us/HT201263

If these steps don't work, contact Apple and ask for help.

https://www.apple.com/support/iphone/contact/

You can find serial number on the box your iPhone originally came with. Also you can use iTunes for it.

From original support article:

iTunes

You can also find your device's serial number in iTunes:

1. Connect your iOS device to your computer.
2. If iTunes doesn't launch automatically, open it.
3. Locate your device. Click the Summary tab to see its information.
4. For an iPhone, click Phone Number to find the IMEI/MEID and ICCID. For an iPad (cellular model), click Serial Number to find the CDN, IMEI/MEID, and ICCID

Find the serial number and other information for your iPhone, iPad, and iPod touch - Apple Support

Anyway the only option is to take your device to the Apple Store for further diagnostics.