Common user errors that ORA-12560 is indication of: 1) improper environment settings or 2) service not started (windows OracleService<sid>).
If that does not help, you would need to provide some background information first:
What do you have? (versions, os, env, etc.)
What does "using cmd prompt to access the instance" actually involve?
Show us your steps and output via copy-paste from command line session.
Edited by: orafad on Sep 26, 2012 10:37 PM
If your OS is windows, ora-12560 is a symptom of the database service not running, check your services applet (Start/Run/services.msc) and check the OracleXETNSListener and OracleServiceXE services are running.
The services may be set to autostart, or not. Without a running database service you won't be able to connect to the instance.