You don't have access TO or FROM the Box? If latter, than give the SMB shared folders details manually in Media Center by Shortcuts. If the previous (+latter), than check if the SMB v.1 settings are all checked in in the Windows Features menu on the PC.
Also give fixed ip's to all your devices, including the PC.
If doesn't work, try to connect with another device, like a Smart Phone and see if it woks.
And try with Total Commander to connect with selecting Network and entering the ip address of the Box.
Firewalls can also block sometimes SMB connection, try it with turning them OFF.