In the case of it having been accidentally removed from the main display (hard to do, but nothing is impossible!!)
Hit the 'EDIT' button on the app's main page, then scroll down to the 'Additional Stats' (these are ones that are available to use - ones which have never been enabled, and ones which may have been inadvertently removed from use)
If Exercise is in the list of additional stats, hit its 'ADD' button and it'll be moved up to the 'in use' tiles.
Whilst you're in edit mode, you can reorder any/all of the tiles to your preferred order by long pressing a tile and scroll it up/down as appropriate.
Hope this is an easy one for you to sort - lemme know how it goes (but I'll just say that if exercise ISN'T in your list of additional stats, you'll probably be advised to uninstall/reinstall the app and set your device up again (and don't worry... as long as your device/app are currently syncing all your data will be safe))
