From everything you describe it doesn't appear that the NAS is at fault - the files are there and you can see it on the network.
It looks more like your computer is at fault. Unfortunately, I know very little about win7 - we don't use windows except for a copy of xp in a vm - but I would start by looking at your firewall settings and anything else that windows update may have changed.
One other thing, if you don't have a backup of your data on the NAS then I recommend you do so, just in case.