I need to know which of these sentences is right ?
My room light doesn't work.
or
My room's light doesn't work.
I need to know which of these sentences is right ?
My room light doesn't work.
or
My room's light doesn't work.
The second sentence is definitely correct.
The first sentence is also correct. In that sentence, room is a noun modifier applied to light:
We often use two nouns together to show that one thing is a part of something else:
the village church; the car door; the kitchen window; the chair leg; my coat pocket;
Much like the kitchen window means the window that is part of the kitchen, the phrase my room light can mean my light that is part of my room.
Both are grammatically correct, except that in the first case you need a hyphen. But I would probably say 'The light in my room's not working'. It is just a case of local usages, personal preferences etc.