My computer doesnt start up

I recommend backing up your files and reinstalling macOS.

You need another Mac to do it, and a Thunderbolt cable. You can buy a Thunderbolt cable at your local Apple retail store.

1. Connect your Macbook to your friends Mac using the Thunderbolt cable.
2. Turn on your Macbook in Target mode. You do that by turning it on, and immediately holding on T on the keyboard. It will turn on with a Thunderbolt logo jumping around on the screen.
   
3. Your Mac's internal disk drive will appear on your friends Mac as an external hard drive. You can open the hard drive and copy all your files over to a safe place like a USB hard drive before you reinstall macOS.

Now, follow this guide to reinstall macOS.

How to reinstall macOS - Apple Support

You might want to take it to the Apple Store and have a Mac Genius look at it.