No idea what exactly happened. Since they disappeared from your computer and the iPad, I suppose you use iCloud PhotoStream to sync your images.
PhotoStream just syncs images, it's not designed to store them. Your pictures will appear there for 30 days, after which they'll disappear, in most cases. There's also a limit to the number of pictures that can be synced through PhotoStream: up to 1000 is possible. If the number of images exceeds this limit, PhotoStream will start removing older images.
The 30 days allow to save the images somewhere else. If you want to store them on your computer, you'll have to save them from iCloud to the PC.
Here's something you can try: if the pictures you lost were included in a recent backup, and if you don't use iCloud Photo Library, you can use this backup to get back the lost images.