It's usually necessary to clean up whats left behind after updating OSX.
Try trashing the prefs. Start by quitting Lightroom 5.
Launch Finder and click the Go menu whilst holding down the Option (alt) key. Then go to Library >> Preferences and clear everything relating to LR5 which usually means deleting the following files:
Then re-launch Lightroom.
Also look for the corresponding files for LR4 and Bridge plist files.
Apple advised a reinstall of Yosemite which I did and everything now working fine. Thank you for your suggestions. I appreciate it.
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