I heard there was a monkey who learnt sign language?

I believe you are thinking of Koko the gorilla, and, yes, she learned American sign language. Her vocabulary contains over 500 words. ^^

Here is the story of Koko the Gorilla on Wikipedia.
http://en.wikipedia.org/wiki/Koko_(gorilla)

Washoe the chimpanzee, was brought up in an environment of sign language ASL. She progressed to telegraphic speech, this is what babies first use when communicating. For example they may say dog to indicate “where is the dog” “look at the dog” and the context usually need to be guessed. Beyond that it appeared that was hoe progressed to syntactic pairs although this has been somewhat disputed. That is Washoe would sign you tickle me, or me tickle Washoe. There was some debate as to weather her preference for order was consistent as would be seen in a child. The however is strong evidence that she did have a preference that could be associated with a grammatical construction opponents point out however that this may be chance and she may not understand how changing her word changes the subject and object of the sentence.

An interesting fact to the side was that Washoe thought of herself as human and when she was moved out of her first researcher home and into the chimp research facility che expressed discontent from being down graded from human status. She referred to the other chimps as “black bugs” also when doing a sorting task where she had photo’s of certain animals eg cat’s dog’s etc and human and asked to sort them into human and aminal categories. She sorted herself as human another chimp brought up with human also did the same thing. She sorted the animals correctly and when she saw a picture of her chimpanzee father she put him in the animal pile however, when confronted with a picture of herself she confidently placed it in the human pile on top of a picture of Rosevelt.

I think what this implies is there is still much work to do in establishing the extent to which chimps use language like us. However I personally doubt that they don’t have a capacity for elements of language and am sure the have emotions and can communicate.