i downloaded IOS 6 last night, I backed up my phone to icloud a few days earlier, however now all my photos have disappeared, can I retrieve them ?

Many users have lost photos while preforming a restore from backup or upgrading the iOS. Recall, only photos in the camera roll are backed up to iCloud. There is a way to turn backups for camera roll on/off, perhaps these users had camera roll turned off.

See:

Settings>icloud>Storage & Backups>Manage Storage, tap the device's name and on the next screen, be sure Camera Roll is turned on.

IMPORTANT...

Photos should be regularly synced to a computer (like you store photos from a digital camera) using either USB via iTunes (on a mac use iPhoto or Aperture to move them to an album) or using photo stream. If you have been doing that, then you can sync those photos back to your device.

If you haven't been saving photos except relying on iCloud to store them in a backup, then that is risky, as many users have discovered. I'm afraid I have no advice in this case - there is no "cure".