A little more detail would be needed to diagnose this properly.
Consider
1. Does the share still exist, check the shares in server manager (if using server)
2. have you changed the Ip address / dns name or dns server recently?
3. if so flush the dns cache on the server and the client.
4. then ping the ipaddress of the server, then ping the hostname to prove connectivity
5. from explorer connect to the server and ensure it is displayed as an available share.
6. if all this works then it 'could' be a user permissions issue but it does not sound like that.
Let me know how you get on.
Please respond with
Platform OS of server (if any) - sharename
Platform (OS) of client
cheers
ed