On many NAS devices a Time Machine share is setup differently to a normal file share. It maybe that when you reconnected to the NAS you only connected to a normal share and not the special Time Machine share.
Other than that your Mac may not be associating itself with the previous Time Machine backup. The articles on the following website in particular this one http://pondini.org/TM/Troubleshooting.html and the 'Reconnecting to your backups' might be of help.
As a first simpler step I would make sure the Time Machine app is copied to your dock at the bottom of the screen as a shortcut, I would then make sure your Mac has mounted the share from the NAS that contains the Time Machine backups. Now right-click or click and hold on the Time Machine app icon, you will now see a drop down menu and in this menu is a choice 'Browse Other Time Machine disks…' via this you might be able to force it to show the contents of the old backups.