There are number of things that could have happened. The images could have been deleted outside of Lightroom. They could have been moved outside of Lightroom. They could have been on an external hard drive that is no longer connected. Without further information it is hard to say. You might want to look over this information to see if it will help you locate some of these missing folders and images:
Within the past few weeks, the folders in my Lightroom catalogues have a "?" on them and the files in the folders cannot be found when I try to develop an image in one of these folders. Why does this keep happening? How can I recover these missing files?
You do not say which OS you are using (MacOS or Windows) and where the images are (internal drive, external drive), but a common explanation for Windows users is that the images are on an external drive, and that the drive letter was changed. That can happen when you connect an extra peripheral, that takes the original letter.