i put my phone to charge lastnight at 20 percent and now its just a black screen and wont turn on i know its not dead cause i charged it all night

See this article: If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support

If your screen is black or frozen

If your screen is black or frozen, you might need to force restart your device. A force restart won't erase the content on your device. You can force restart your device even if the screen is black or the buttons aren't responding. Follow these steps:

• On an iPhone X, iPhone 8, or iPhone 8 Plus: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the Apple logo.
• On an iPhone 7 or iPhone 7 Plus: Press and hold both the Side and Volume Down buttons for at least 10 seconds, until you see the Apple logo.
• On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Home and the Top (or Side) buttons for at least 10 seconds, until you see the Apple logo.

Hmmm... Did you see the whole article? It's also possible that your charger wasn't actually charging the iPhone, although that shouldn't make the iPhone COMPLETELY unresponsive. Worst case scenario, contact Apple Support (you don't have to choose a phone method—I personally prefer using the in-browser chat menu, for example): Get started

Also, you could take it into the Genius Bar since Support will probably just point you towards the same article I already mentioned: Genius Bar Reservation and Apple Support Options - Apple

No problem! Let me know if they can fix your problem! 🙂