Cognizant Technology Solutions — Cancellation of the candidature after accepting the offer letter and serving my notice period

Address: Cognizant , Kolkata

There is a serious legal issue which happened with me. After accepting the offer letter and serving my notice period I suddenly received a call from talent acquisition team that my candidature has been cancelled. On asking the reason to HR she is telling she is not aware and recruiter is not even responding. Is this a good proffessional ethic expected from such a giant like Cognizant. I just want to know the reason for my cancellation of the candidature. Please look into this else some legal actions needs to be taken