Lost photos

iCloud does not delete or touch photos in the Camera Roll or photos transferred from your computer.

With Photo Streaming enabled, iCloud will add photos that were captured by another iOS device with the same iCloud account and transferred to the Photo Stream with a supported app on your computer such as with iPhoto on a Mac.

Which photos have disappeared - photos in the Camera Roll or photos transferred from your computer?

If photos in the Camera Roll, photos in the Camera Roll can and should be imported by your computer as with any other digital camera. Photos in the Camera Roll are also included with your iPhone's backup. Is your iPhone's backup being updated on your computer via the iTunes sync process, or is it being updated wirelessly with your iCloud account?

Photos transferred from your computer should remain on your computer.

If you have lost your photos from your camera roll you can restore from a backup as long as the backup has them. If they are Photos that you synced from your computer try connecting your iPhone back and sync them again.

Did you not import these pics to your computer regularly as you would with any digital camera?

Camera Roll is temporary storage. You should be importing them regularly.

If you have failed to do this, then they are likely gone. You can try restoring from backup as ataylor suggested.