Windows Network Discovery has been a pain of late. I have my Workstation set up as a Client of my HTPC, and for some time visibility of the HTPC in Windows Explorer from my Workstation has been quite erratic. No amount of refreshing the Explorer display will make my HTPC show up. I suspect that Microsoft has turned off some of the Network Discovery functions or something, in an attempt to protect us from ourselves, or broken something. Note that my Workstation based MC Client still sees the HTPC MC Server Library no problem, and I can still play media files from the Server on my Client.
My solution is simple. If my HTPC was actually named "HTPC", then entering \\HTPC\ in the Windows Explorer address bar will typically show a drop-down list of shares available. If that didn't work and I knew I had a shared folder called "Music", then I would enter the address \\HTPC\Music\ and Windows Explorer would connect. Once it connects to one share, it seems to show all shares, until I close that Windows Explorer window, when it stops working again.
Not a fix, but an easy workaround. A bit of a pain though. It could be a Firewall setting on my HTPC. Usually if both PCs are told their network connection is a Private network and should be trusted, Network Discovery just works. Not now.
I had to reset the Windows Defender Firewall on my Workstation recently to fix an issue caused by a Windows update. Maybe I need to do that on the HTPC as well. I don't even use the Windows Defender Firewall, but it still broke my Workstation!
PS: You probably had the 32bit version of MC still set to start with Windows. It started first, so the 64bit couldn't start, since a second instance isn't allowed.
PPS: Any network shares on your NAS should still be working, using the above method, or if you have a mapped drive to the NAS.
PPPS: I understand your media files are on the NAS. Where are you library files actually stored? On the C: drive of your PC, usually in "C:\Users\[UserID]\AppData\Roaming\J River\Media Center 23\Library", or are they also on the NAS?