Why do pictures disappear from Photos on my Mac mini?

Hard to tell from here. You didn't give us much information to work from.

Do you use iCloud Photo Library to sync content across devices? If so, did you perhaps delete these pictures on another device?

Do you share an Apple ID or a Mac user id with another person? If so, might that other person have deleted these pictures?

Do the pictures disappear while you are using your Mac mini or between uses?