Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
It is the password ***** the account you sync the calendar too. It is normally your email password. Enter your email password ***** does it accept it ? If not, try your iCloud password.
If it still not working, then you need to delete your email account from your iPhone and add it back. Go to Settings > Mail, Contacts, Calendars. You will see all the accounts added to your iPhone in there. Tap account one by one and delete it. Then add them back.
If you don't remember your email password, please let me know what your email address is and I will send instructions to reset it.
Please let me know if that didn't work, so I will continue to help you