• Legal actions for not serving the notice period.

Dear sir,

 I was working with a reputed company only for 5 months. I gave my resignation to my boss and go for work extra 4 days as a notice period, after that I left the job. After 6 months later i receive a letter from the Head Office that I have to pay for the notice period. But I was not a permanent staff of the company, I was in probationary period. My question is what legal actions can a company take against the employee in this situation??? What to do?? Please help me out..