Hi Susan,
A wonderful student group from the University of South Florida created a [[Amazon Fire HD 8 Troubleshooting#main|troubleshooting page]] to help you identify what's going on with your device. If you find that your screen needs replacing, you can find the part [https://www.ebay.com/itm/Amazon-Kindle-Fire-HD-8-5th-SG98EG-Screen-Replacement-/361766554981?rmvSB=true|here] and all of the tools needed in the iFixit parts store.