It's fixed now. Viber caused camera crash. That means if I start this mobile, then the camera works, skype works (camera) and other applications too.
After a video chat with someone, camera does crash and all other applications which run camera crashed too. After a restart camera worked again.

Fix:
Remove Viber
Download an older version of Viber from the Internet (apk file)
Copy and place the file in your mobile
Open File manager
Open the apk file and install the apk file
Disable auto-update of Viber