I had this issue before. It happened to me one time. It seemed to clear itself up after a reboot and a charge. I rebooted the phone end the camera failed. I plugged in the charger and it started to work when it was plugged in. Then I unplugged it and it worked. That was the only time I've ever seen that though.
Have the same issue. My Note 2 was okay at first the steps i followed was real. But when i download an app called "Otaku Camera". Well, It happened again. My camera was not working and it keeps on popping "WARNING: Camera Failed". I went to Samsung Service Center at first they told me it just need a software update but i've done that. But, nothing happened. They told me that some apps caused that issue. That some apps corrupted the device especially the Camera. Now they are replacing some parts with my phone. :/ And it cost me a lot. BIG TIME. so beware. Some told me check the reviews well. But, even reviews lie. WHAT A TOTAL BUMMER SAMSUNG especially GOOGLE PLAY. So yeah!, GoodLUCK.
1. Connect your device to your laptop using data cable
2. Go to folder: Computer\SAMSUNG-SGH-I747\Phone\Applidium Image Cache
3. Delete everything in this folder
4. Go to folder: Computer\SAMSUNG-SGH-I747\Phone\Android\data\com.sec.android.gallery3d\cache
5. Delete everything in this folder
6. Restart your phone
7. Camera works