calendars stopped syncing

Plese provide more information. What devices are you trying to sync? Which of these device(s) are not syncing changes made on the calendar on the device to your calendar on icloud.com?

On your phone, go to Settings>Mail,Contacts,Calendars>Default Calendar. Here, make sure you have selected an iCloud calendar as your default and not something else like On My iPhone, On My Mac or a Google, Yahoo or exchange calendar. If you haven't, make this change, then enter a new event on your phone and see if it syncs now.

If your settings are correct and it still isn't syncing new events, enter any events from your phone that are missing on your iCloud calendar on icloud.com so the icloud.com calendar it is up to date. Then go to Setting>iCloud on your phone, turn Calendars to Off, choose Delete from My iPhone when prompted, then after a minute or so turn Calendars back to On to sync back the iCloud calendars to your phone. Then enter a new event on your phone and see if it syncs now.