For Patient Accounting and Advanced Patient Accounting: 

Close and re-open Medisoft.  Advantage Server is built into these versions of Medisoft and restarting Medisoft will essentially restart Advantage.  Restarting the computer should also clear out any cached issues.  Re-installing Medisoft on top of the same version may resolve the issues if just restarting Medisoft doesn't.

For Network Pro:

1. Check Network Connection, verify you can open the shared folder on the server

2. Verify the Advantage Database Server Service is running on the computer hosting the Medisoft data.  Use the Advantage Configuration Utility on the hosting computer to review the Advantage Database Server Service's status.

3. Verify Firewalls are not blocking the Advantage Database Server (\AdvantageInstallFolder\Server\ads.exe) on the hosting computer and that Medisoft is not being blocked on the client computer.

4. Try adding a ADS.INI file pointing to the server to the Medidata Root folder, the client's Medisoft Install Bin folder, or the system root folder (windows root directory)

SyBase also has a more in-depth document on resolving 6420 errors on their Advantage KnowledgeBase:

6420 Unable to "discover" the Advantage Database Server

Use an ADS.INI file to resolve a 6420 error

Advantage 6420 Solution Summary (with links to detailed walkthroughs)