Here is the simplest way how to know whether oracle instance is up or down :
1. Try to connect to oracle using sqlplus /nolog
2. And then put code conn /as sysdba
3.  Let see in v$instance using query : select status from v$instance;
If the result is OPEN, it means that the Oracle is already up but if oracle give an error mesage message
ERROR at line 1:
ORA-01034: ORACLE not available -> oracle instance is not running.
4. If oracle is down and then You want to start it, You may using command startup and wait until database mounted and opened.