Not necessarily corrupted (that's actually pretty hard to do). Try these steps:
- Log onto the database server
- Set ORACLE_SID=yourSID
- at a command prompt, type sqlplus /nolog
- in sql*plus, enter: SHUTDOWN ABORT, then STARTUP
- If the database starts up normally, issue a SHUTDOWN IMMEDIATE and another STARTUP.
If that starts the database, you're good to go. If not, you should then get some sort of error message that will get you started down the path of getting a solution. Check the alert log for your instance for additional information. Update this question if you still need help.
Assuming the manual startup fixes the issue, then to make your database startup automatically when the service starts, edit the registry. Look for the key HKLM\Software\Oracle\HOMEx\ORA_sid_AUTOSTART. You want the value of this key to be TRUE. Note: HOMEx will be of the form HOME0, HOME1, etc.