What is AppleCare+? How is it different from the warranty Apple products come with?
AppleCare is Apple’s term for its warranty programs. Every Apple device comes with a certain stretch of AppleCare warranty service and phone/chat support for free, typically one year for the warranty and 90 days of phone support. You can pay to extend that protection for longer, which is called AppleCare+.

No. If you already know you want it, buying AppleCare+ with the product is a good idea, just so you don’t forget. When you’re shopping at the Apple store and Apple.com, you’ll be offered AppleCare+ at checkout, and authorized Apple resellers can offer AppleCare+ too.
But you have some time. Generally you get 60 days from the purchase date to buy AppleCare+ (only 30 days if you’re in Japan), and Apple has a page where you can check your exact device’s eligibility.
Still, to buy AppleCare+ after the fact you generally have to run a diagnostic test on the device in question, so Apple knows it’s in good working order. And no matter when you buy your AppleCare+ plan, the coverage starts from the day you bought your device. So you don’t get an “extra” two months of coverage if you wait two months to buy the plan.

Charge Your Phone for 30 Minutes.

Restart iPhone, if charging the battery didn’t turn your iPhone on, the next thing you should try is to restart the phone. To do this, hold down the on/off button at the top right corner or the right edge of the phone for a few seconds. If the phone is off, it should turn on. If it’s on, you may see the slider offering to turn it off.

Hard Reset the iPhone: – Try a hard reset if the standard restart didn’t do the trick. A hard reset is like a restart that clears more of the device’s memory (but not its storage. You won’t lose data) for a more comprehensive reset. To perform a hard reset:
If you have an iPhone 8 or later, press and quickly release the Volume Up button. Then press and quickly release the Volume Down button. Finally, press and hold the Side button until you see the Apple logo.
If you’ve got 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.
If you have 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.
Continue holding them for at least 10 seconds (there’s nothing wrong with holding for 20 or 30 seconds, but if nothing has happened by then, it probably won’t)
If the shut-down slider appears on the screen, keep holding the buttons. When the white Apple logo appears, let go of the buttons and let the phone start up.