Legal Notice of joining competitor and blame of sharing data

Sir, I have joined as Trainee and worked in MNC (M/s X) with full hardwork and honesty for 2 years on rotational basis in different departments. 9 Months back, I came to know some unethical practices , made by department head for taking promotion and advantages. He harrassed me mentally for one month like staying late in offices, and then one day asked me to resign immediately from my services. and threatened me if I dont then he would ruin my relieving and career. I followed the same. And left the company with clearing all my dues and handing over all the data to boss. But they did not provided me with relieving letter till date. Due to this I was jobless for approx. 6 months. One month back , I joined the competitor company dealing same customer as of M/s. X. Today , M/s X company send me legal notice blaming of stealing confidential datas like design, quotations, drawings and sharing with newly joined competitor company. And asked me to return all data and give written apology within 7 days. Being honest, nor I had any confidential data and neither I have shared any data with anyone. Kindly suggest how should I reply to this notice. ? Also after five months of my leaving the company M/s X. H.R head informed me submitting a big amount to take my relieving letter. After that I addressed this harrasment issue to Country head. But 2 months have passed and no reply have been recieved. Also suggest how can I initiate legally about this issue, I have all communications on my email.