Clips turned black

You need to provide details for anyone to help
Mac or Windows and exact version of the operating system
Premiere Elements version
CPU brand and model and clock speed
Video adapter brand and model and driver version and amount of video ram
Number and size and type of hard drives
What kind of video are you editing?
-hint 4k video on an underpowered computer will not play well
.
For example, what I have (home built July 2014)
Windows 10, currently at version 20H2 - Premiere Elements 2021
i7-4930k 3.4Ghz 6 Core CPU on Asus Sabertooth X79 Motherboard with Corsair 2x16Gig Ram
Raidmax Gold Power http://www.newegg.com/Product/Product.aspx?Item=N82E16817152044
Samsung 256Gig SSD Boot Win10 64bit Pro and ALL program installs, about 80Gig is used
Crucial M550 512G SSD for all input... video files and pictures from camera
Crucial M550 256G SSD for temporary and video project files
Crucial M550 128G SSD for all exported output video files
MSI 2Gig GTX760 video adapter, nvidia driver 432.00
I edit 1280x720 video from a Canon SX510 camera, and it plays 'as smooth as butter'

Your clips turned black or your preview of the timeline shows only black?

If you double-click on a clip, does it show up as black in the Clip Monitor window?

If you output your movie as an MP4, is that video black?