Expat Properties — No handover of property after taking full payment, Not responding to calls and emails

They took full amount towards sale of apartment in 2015 and till date have not started the construction. They do not attend the phone calls neither reply my emails.
There is no one responsible whom I can talk to most of the time the office is unattended if luckily someone picks up the phone they say will pass on the message to the concerned person and they will get back to you but it doesn't happen.

Can someone give me an idea about the legal process.

Thanks