In normal situations, you just need to wait until the initialization process ends.
But this could also happen if something fails during the database initialization process, for example, corrupted log files or disk errors.
Try running this at the server:
SQL> conn / as sysdba
SQL> shutdown immediate
You will probably get errors here, which wil tell you what the problem is.
If you can’t run the above, look for information in the alert log, which should be located in: