Fake or real noice, what action to take

I was working in a company, their I newly joined, I was not able to handle their harassment and I resigned the job after one month but they did not confirm my resignation,I waited for 15 days but no reply. Then I moved to my native. I did not took any assets from that company. And now they have sent a notice saying that to pay fine and return the assets. And the notice was in a plain letter head, with some signature and without seal. What to do I don't know, can you please suggest me.