Failed to connect to oracle SQL Plus I tryed to connect to SQL Plus as the admin "System", I'm sure that the password I wrote was correct but the connection failed. I also tried to connect with Scott/tiger but it failed as well.
What can be the problem, and how to resolve it please ?   

this is what I get after trying to connect using both System and Scott:
Restart your database and that may clear up your issue.  Any more relevant advice will require alert logs or trace files from the database.