My computer sound doesn't work?

i went to CONTROL PANEL and CLICKED "SOUND" it showed me "PLAYBAK" , "RECORDING" , AND "SOUNDS" in "PLAYBACK" it says "SPEAKERS (WORKING)" on "RECORDING" it says "FRONT MIC (NOT PLUGGED IN) , MICROPHONE (NOT PLUGGED IN) , AND LINE IN (NOT PLUGGED IN)? What does that mean? Help please! (No sound comes out this just happened recently!) it's a DELL Computer!