Cause: It is believed that some times due to some environment issues drives might not be found by operating system on startup time.
Fix: One should be looking to fix the underlaying cause then on how to get these shares back but still I would mention how to get these back after machine is already rebooted.
To get the admin shares re-created by MS Windows please restart "Server" service from services.msc (services applet)
Server service has another service called Computer Browser that is dependent on it. In case you are using command line e.g. "net stop server" please remember to start back Computer Browser service back again.
