Hello and welcome to the forum,
Does every PC on the network have the same username? I'm guessing that you dont have a password on the other PC's which have the shares on?
If so, add the same password to the other PCs and this should work again. When your computer attempts to access a share it will use the current username and password of the PC that you are logged on to as the default authentication credentials. So when you access a share it uses these credentials.
Another alternative is to make sure that the everyone group has full permissions on the shared folders on the other PCs. This needs to be done in both the Security tab and the Permissions tab within advanced sharing.