We're sorry about the trouble experienced with Lightroom. We're here to help.
Which version of Lightroom and operating system are you using? Is this happening with a particular file format? Where are the images located? Also, which Graphics card did you install on the machine? Is it up-to-date?
Check this article for the troubleshooting steps: https://helpx.adobe.com/lightroom-classic/kb/photos-appear-black-gray-rectangles.html.
Let us know if this helps.