There is a simple checklist to follow.
1. Make sure the card works (no exclamation points on it in your device manager).
2. See if the light in the back of your card is lit up.
3. Make sure you have the correct protocols installed.
4. If you are using TCP/IP, make sure you have the correct IP address, subnet, etc.
5. Make sure you are in the same workgroup.
That should do the trick.