Need Social Security Number but have left USA....help

Hi I moved to California from the UK in the summer with work. They used a temporary SSN number in order to pay me my salary while my new one came in the post. However, the job fell through and I returned to the UK before my new SSN arrived. Now my work (which I have left) is asking for my SSN.....which I never received having left. Please can someone tell me how I go about getting it? I've tried calling the SSN help line but the automotive options are not relevant and the call ends with it saying they are very busy. Thanks in advance