You need to tap on the home button two times and do it fast. You’ll see a list of the apps you have been using most recently. Find the camera app, and swipe up to close it. Start it again.
If the previous solution did not help, again, tap on the home button two times, and once you get the list of the apps, by swiping up, close several more apps.
If you have recently downloaded some new app, and if from that moment the issue with the camera pp stared, delete the recently downloaded app, because it may be faulty. Find its icon on the Home screen, tap and hold your finger on it until you see an X, click on it, and the faulty app will be deleted.
If there is a new iOS version, make sure to get it. The problem with frozen apps is always software related, and the update can solve it.
Backup your files to iCloud first. Navigate to Settings, General, Reset, and tap on Erase all Content and Settings. When the content and settings are erased, you just need to follow the steps to set up your iPhone 6 as a new device.