Look at your settings. Go to Settings - Mails, Contacts, Calendars and look at the account which you use for the Calendar app. Make sure the password is entered there. If this doesn't help, delete the account and reinstall it.Every time I try to put something on my calendar it asks for my password. It did not always do this so how can I .stop it asking for password. Sorry I don't know how to find a thread which already has this question
Look at your settings. Go to Settings - Mails, Contacts, Calendars and look at the account which you use for the Calendar app. Make sure the password is entered there. If this doesn't help, delete the account and reinstall it. If this doesn't help either, please provide more information. Did you set up a CalDAV account (calendar only), or do you use an email account for the app? Which one? Did you change anything on this account lately?