This is mostly a user-community and so not all posts will get addressed by Sonos Support Staff, who are limited in number. You perhaps may find it best to contact the Sonos Support Staff direct via this LINK instead.

Otherwise, if you need the assistance from other Sonos users here, you really need to post much more information about your setup, such as:

• The details of the Controller device used and free space available. 
• Details of Firewalls/VPN and or security software on the controller device (other Apps running in background, if relevant)
• The Sonos App version being used - S1/S2 and if it’s updated to latest build.
• If you are trying to connect to http/SMB share? (what SMB version is library using?)
• What the library path is, (if using the IP address, or netbios name)? or…
• The make/model of NAS and…
• Size of library/file types etc.
• Whether you have tried perhaps already rebooting your network router, controller device, the Sonos associated product etc. 

Is the music on a Mac? Reddit has a few recent posts which indicate Mac permissions issues with the file sharing, which provokes a hang in the controller.