Suddenly cannot connect to database

Oracle Database 12c Release 12.1.0.1.0 - 64bit Production

After running Oracle 12c flawlessly for quite some time, suddenly we can't connect to the database. There are nothing new had been installed and configured on our Oracle server.

When I tried to shutdown the database, I got this error:

SQL> shutdown immediate;

ORA-01034: ORACLE not available

ORA-27101: shared memory realm does not exist

Linux-x86_64 Error: 2: No such file or directory

so, I tried to use this method:

sqlplus /nolog
conn sys/sys as sysdba
shutdown abort
startup

it run well, but the next week, it happens again, and I restart it again.