The camera is not working and your screen is blanking out very quickly. You are probably in power saving mode. When your phone is on its last legs it will usually switch to power saving mode. However, you may notice that when you insert a new battery, the phone still remains in power saving mode.
To deactivate power saving, press the power button shortly and press up on the navi key to Deactivate Power saving. If it says Activate Power Saving then your problem is not power saving mode.