Since Database 12c Enterprise Manager DBConsole has been replaced by Database Express. First try to work with, ended up with an error like this:
Take a look into AlertLog of the database:
Exception [type: SIGSEGV, Address not mapped to object] [ADDR:0x5] [PC:0x2CA8CC2, qervwFetch()+338] [flags: 0x0, count: 1] Errors in file /opt/oracle/app/diag/rdbms/cdb12c/CDB12C/trace/CDB12C_s000_4267.trc (incident=67380): ORA-07445: exception encountered: core dump [qervwFetch()+338] [SIGSEGV] [ADDR:0x5] [PC:0x2CA8CC2] [Address not mapped to object]  Incident details in: /opt/oracle/app/diag/rdbms/cdb12c/CDB12C/incident/incdir_67380/CDB12C_s000_4267_i67380.trc ... found dead shared server 'S000', pid = (22, 2)
BUG 17071138 – ORA-7445 [QERVWFETCH()+338]
–> Base BUG 16527374 – [12100-LIN64-130318]CDB:ORA-7445:EM EXPRESS HOME GOT IO
Oracle short after 12c release date a patch BUG 16527374 has been published (13.07.2013) the problem will be fixed.
–> Notice: The Patch is ONLINE installable –> see here