I meant using Infuse. Can you still play the files via Infuse even though it’s not indexing? If not then it sounds like maybe your NAS IP may have been changed by your router or something else changed the LAN name/IP.
Are you using a name like “MyNAS.local” or are you using an IP like 168.192.1.100?
The fact that the Finder can see the files and play them doesn’t have to indicate that the file system on the NAS is all good. Synology OS surely has a feature to re-scan it’s volumes. I have QNAP systems and I have seen issues during file re-scans in the QNAP system that hasn’t yet impacted the use. The same is true for more or less all computers, macOS, Windows etc. Just do it…
I ended up removing the Movies and TV Show folders from Infuse. Then I pointed the Infuse app back to those same folders on my NAS. Infuse brought them all in, but the downside was that it had to reload all of the metadata which took some time.
This still leaves me puzzled as to why the original folders suddenly refused to index.
Oh well, I’m glad it’s resolved, until the next time!