We have chased these types of issues time and again and it can be any number of things that are causing the issue. I'll give you the top three in no particular order
1.) You have changed your username and or password for your computer account and did not use the NAS's web interface to make the same adjustments to reflect the changes in access permissions, with respect to the shares on the device. Each NAS is a bit different but your documentation should provide some help with respect to what you might need to do.
2.) Some change or adjustment in the network adapter settings have changed and "Enable NetBIOS over TCP/IP is no longer checked under the WINS tab, within the properties of IPv4, in the properties of your network adapter. Or something has changed with respect to the relative services (services.msc) necessary to support file and printer sharing, so.....
Code:
type
services.msc
into the search box and hit enter
confirm that the following services are started, consider setting the startup type to automatic
Computer Browser
Function Discovery Provider Host
Function Discovery Resource Publication
Network Connections
Network List Service
Network Location Awareness
Network Store Interface Service
Server
SSDP Discovery
TCP/IP NetBIOS Helper
Workstation
Also check the network and sharing center and confirm that your options are configured as to support sharing on both machines.
3.) Could be third party firewalls (ZoneAlarm, Comodo, etc.,) or any number of Internet Security Suites ( AVG, Avast, Avira, Norton/Symantec, McAfee, etc.,) try removing any such product by using the program uninstaller if available in the programs directory, or the Programs and Features applet in the Control Panel, follow that up with the vendor specific proprietary removal tool, a list of some can be found here
Security Software | SingularLabs if yours is not present in the list, try googling for the removal tool.
Consider replacing any such product with the built in Windows Firewall and
Link Removed due to 404 Error
One final thing that sometimes helps with these types of issues regards the Local Security Policy
Click start and type
secpol.msc
hit enter
See attachment