No Audio Output Device is Installad

Hello, im also having problems with my computer volume as many other people. On the bottom right hand corner there is a speaker pic with a red cross on it and when i put my mouse over top of the pic of the speaker it says 'no audio output device is installed' and just about two days ago it worked fine but time after time this problem keeps occuring and than stops the next day but the problem wont go away now. I dont know what to do now. PLEASE HELP ME WITH THIS PROBLEM! PLEASE!!!!!!!!!!!