Reboot of the Oracle system during normal maintenance does not trigger Oracle instance to run the STARTUP command.
Run the following command to enable auto start mode for the PROTECT side. Update the sid value if not using protect.
ORADIM.exe -EDIT -SID protect -STARTMODE auto -SPFILE -SHUTMODE immediate -SHUTTYPE inst
You may also want to check if under
HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\your_oracle_home_name
that ORA_your_sid_AUTOSTART is set to TRUE. Example: