Some time network printers are not accessed from your computer. While they are accessed from other network clients.
Your computer is actually not accessing the network computer through which the printer is attached.
Access that remote computer first by following these steps:
1) Click on My Network Places Icon displayed on your desktop.
2) Double click on Entire Network Icon
3) Double click on Microsoft Windows Network Icon
4) Double Click on your network icon
5) Double click on that network computer withwhom printer is attached
6) Enter Username and password and press ok button
You can now access the printer which you were desired to access a few seconds ago.