Hi Dan,
This is because Moodle does not recognise it as being on that page.
Which Theme are you using at present, as this may well be where the problem started, especially if the change of version of Moodle has lost the data related to this specific block.
As it is visible on the page then set the page to Edit, then Edit the block's settings to be on the side you wish it to be as well as the position you what it to be in on the page. Then save the settings. This should hopefully reset the block.
Cheers
Mary