A couple of suggestions.
Trash the LR Preference file.
Create a second User Account and test.
There was another post similar to yours and the import worked with a different user account.
Last option uninstall LR and reinstall.
IMHO you never know what is going to happen when you upgrade OS X.
If it Ain't broken stop trying to fix it. That is in reference to upgrading the OS when you really don't need to.
If there is no error message go to Lightroom Preferences> General Tab> "Reset all warning dialogs".
Try again and see if an error message is displayed.
Thanks for the suggestion, sadly, this didn't do anything - still no error message. Literally no nothing. :-(
Thanks Just Shoot Me... have definitely uninstalled and reinstalled already. And wouldn't usually update system, but it was a last resort... LR stopped working BEFORE I updated it so wondered if there may be an issue with the OLD system... :-(