@Sakin: Thanks so much for your reply!!! Had looked into the plugins and disabling the Jetpack solved the whole issue immediately! It seemed that the plugin needed to be updated & now with the new version of Jetpack, it seems to be working all good.
It was really frustrating that almost the whole admin site wasn’t working at all but learnt a lesson that all the plugins as well need updating when updating WordPress version.
Thanks once again for your swift response, much much appreciated!