Could it be my clutch and pressure plate or my gearbox?

I was driving and stopped at a traffic signal, as soon as am ready to move i depress the
clutch pedal and select 1st gear, i release the clutch and started to move when i heard
like the vehicle jumped out of gear, i checked but it was still in gear and not moving. I  
could hear a little rough rotation underneath the vehicle. IT still goes in and out of gear
smoothly when i depress the clutch. just not moving.