You left out some necessary information to help with your problem.
- What role is the server in question fulfilling (domain controller, file server, application server etc.)?
- What OS is the client using?
- How are you mapping the drives? (Are you using the users credentials and just connecting to the server or specifying credentials manually using
net use * /user
?)
If the server is a domain controller, it's likely that the tokens used for authentication are expiring when the DC is rebooted. That is my best guess. If the server is just a member server then I don't know. If the tokens are in fact expiring, then you would need to logout and log back in on the workstations for the drives to reconnect. Check your security and system logs to see if you see any weird events after doing the restart. More than likely the answers are in there.