Someone took property fraudulent by taking my signature

I have a land which is very costly someone want to takeover it.At that time by mistake I signed Something which is made on a white paper sheet and by mistake I forget to read whats written over there and using my weakness he also take my 10 fingerprints by something electronic material .After some days I heard something that my land is sold but till there is nothing happen on my land.Now I scared because to whom I made the agreement he stop peak up the phone and still its continuing.So please tell me that what's step should be taken by me right now to protect my land from property fradulent ??