Headphone volume too low on DDJ SR

Hi I Purchased a DDJ SR about a week ago and everything has been fine until last night. I am staying with family so I have just been listening through the headphones and have not plugged in the output. Last night, the volume level dropped quite low and I cant get it to come back. Have switched USB ports, checked that I have the latest firmware, and adjusted all of the level knobs to their original position. I am using a MacBook Pro with OS 10.10.1 and Serato 1.7.2 . Any suggestions?