Employer reduced my salary without notice, is this legal?

I work in a tech startup in Bengaluru. Last month, my employer suddenly reduced my salary by 20% without any prior notice or explanation. I've been with the company for over two years and this has never happened before. I’m really stressed because I rely on my full salary to pay my bills. I tried speaking to HR but they avoided my questions. Is this reduction legally allowed and what steps can I take to address this?