Hello gwendoline1999,
Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you're having issues with your iPhone going black. Try the steps here to see if they resolve the issue: If your iPhone won't turn on or is frozen - Apple Support
If you need service, we recommend calling the location that you choose to make sure they're open. You can find an Apple Authorized Service Provider closest to you to make a reservation, and have the repair done if you go to this link: Find Locations
Check out this article for more information on what to do before you take your device in for service: Get your iPhone, iPad, or iPod touch ready for service - Apple Support
We hope this helps.
Cheers!