Got an offer letter. Due to delay in submission of ducouments, no job.

I applied in a company last recently. I had given 2 to 3 interview round. Thereafter they said that they will send me offer letter in mail on monday. They asked to submit some documents and i had 2 documents less. I told them that i will get documents and soon submit it then join. I got those documents in the weekend gave it to HR. Now they are telling me that the position is already filled. I have the offer letter but i accepted it only over the phone. what can i do about it?