"open with photoshop" missing from the 'open with' list in Bridge

(Adobe Creative Suite CS 5.1 + Win 10, with the latest updates) - In Bridge, when you right click on an image to open it  in Photoshop the app is missing from the list of applications to use. This is a recent issue and I have no idea what may have caused photoshop app to disappear from the list. Question: is there a way of getting Photoshop back on the list of apps that are available to open the image?

Thanking you in advance,

Rory Tate