Samsung Galaxy S4 I9500 camera not workin problem creates when camera strip damage or hardware problem in cellphone motherboard. First confirm that mobile settings are correct before disassemble mobile. Reset mobile settings or wipe all settings to its factory default settings. Here is the trick how to reset samsung glaxay s4 mobile.
How to Hard Reset Wipe Samsung S4 Adwance Mobile ^(http://www.gsmhoster.com/2014/06/samsung-galaxy-i9500-s4-pattern-lock.html)
If problem not solved with hard reset trick then disassemble mobile and check camera strip or replace camera. Here is a diagram in which all points are traced related to camera. Check all points with multi meter and make jumper any damage point as shown in diagram below.
Samsung I9500 Galaxy S4 Advance Camera Problem solution diagram.
Samsung I9500 Galaxy S4 Advance Camera Problem Fixed