iPhone 6 stolen iCloud accessed
Depending on the configuration, an attacker with access to iCloud may get:


Contacts.
Calendar.
Notes
Safari bookmarks, navigation history, reading list.
Reminders
Passbook contents.
Location of other Apple devices associated with the same account, with Find my iPhone on.
Photos taken with any Apple device associated to the same iCloud account, or shared with that account (if iCloud Photo Sharing or iCloud Photo Library was on).
Access to any document stored on iCloud (Pages, Numbers, Keynote, others).
A full backup of any iOS device stored on iCloud (including all information on any app installed up to the last backup made).


Change your password immediately if you haven't done so yet.

Having your iOS device stolen and having your iCloud credentials compromised are two unrelated events, unless you happen to have your device unlocked, without password or pin, and have your iCloud password written in plain text somewhere inside of it.