I need to gain weight.. simple advice please?

i had met with a major accident 4 months back. i was 72 kgs for a 6 ft height which was avg. but after the accident i lost 7 kgs which i am unable to regain and now i look underweight. can some1 give very simple advice on how i could regain my weight (i need 9 kgs atleast now to become 75 which is normal).