Why is the audio not working on OS X Yosemite?

I have installed OS X Yosemite on my 2 computers. One is working fine but the other doesn't show any audio output devices and can't change the sound effects also. The startup sound is still there. I have tried few things but nothing is working for me. I don't want to backup the data and reinstall because I have some important software which I don't want to reinstall. Please help me to solve the problem.