Joining competitors clause

I was working with an NBFC. there was a clause in agreement not to join a competitor for 2 years after leaving job. But i joined anther NBFC after resign previous company but with proper 1 month notice period and also got provisional relieving from hr. But company sent me a notice through its laywer that i hv joined competitor against agreement clause. They said you have taken their client data and passed it to competitor but it is not true and they hv no proof of it. They wrote in notice that company faced monetary loss and i have to pay for that and leave my job in 14 days otherwise they will take legal action against me. Plz give suggestions.