If your grandparents have left the home on their own and if it is believed that they have not been pressurised to leave the house or have not been kidnapped by anyone,  then your father cannot do anything about it,  especially if they are mentally sane and do not have any mental illness. 

If your father suspects them to be missing under any miserable circumstances,  he can lodge a criminal complaint with the local police and a man missing case can be registered. 

File a complaint before near police station immediately . The same time put a missing add on news papers

First you need to check where they have gone. If not found immediately lodge a missing complaint

File missing police complaint as all mobiles are switched offa d they cannot be contacted in any manner 

Your father must lodge a police complaint against that relative for kidnapping and abduction 

Immediately lodge complaint of missing persons. 

Once they are found out its their choice whether they would come back to you or not. 

Dear Sir,

You may immediately report the matter to police and get FIR registered either as missing complaint or as kidnapping by such relative. Such relative will be arrested.

Hi, try to find out them. It is better you can lodge police complaint in the nearest police station .

1. Try contacting all your mutual relatives who could possibly be in their touch.

2. Lodge a complaint in the nearest police station communicating all the events and request the Station House Officer to assist in locating them. 

Act as swiftly as possible. In such case a complaint must be lodged immediately.  

Why did they leave you?? I mean were there any issues? Lodge a missing persons report immediately.