Mac or Windows? What version?
What exact version of Lightroom?
Assuming that in the Library grid, your photos have the "missing" icon ("!") showing, in the Folder panel do any of the folders get marked as missing ("?") ?
This usually happens when the photos are on an external drive or network connection and the drive or device name of that external drive has changed or you might have moved the images from one location to other.
Please check the below link to find the original image.