Hello Rajesh Rajan,
There are so many reason to occur the issue you mentioned
1) Virus/ malware infection
2) Contact database corrupted
3) Due to 3rd party application which share your address book such as Viber, line ets..
4) Sim card contact memory location having issue with data filed
5) recently you imported contact complicating or address book limit exceeded
Simply you can reset the device and most probably fault will be fixed but before reset the phone make sure all contact backup to the your google account.
Hope this answer help you!
Thanks