Oracle automatically unmount
Ok, that error (ORA-00600 [6856]) is related to corruption.

Normally a DBA would go to My Oracle Support, follow the notes (for example    ORA-600 [6006] ORA-600 [6856] During Startup Instance, Followed by Termination by SMON (Doc ID 549000.1)), identify the affected objects then try to repair them, recover corruption, or restore a valid backup. 

If you can not do that, and this is a sandbox/development database with no important data, in my opinion the easiest and fastest solution for you would be recreating your database with DBCA (Database Configuration Assistant).