18 Month Old suddenly hysterical at bedtime

[mairimac]

My DS who is 18 months old has virtuallys slept through the night since 8 weeks old. He has always fallen asleep on his own and we've never had any problems, he even used to ask to go to bed. However this last week every time we put him to bed he gets hysterical. There is nothing wrong with him because if we go in the room he sits up laughing and chatting to us. I'm trying the crying method now as I'm so tired and frustrated. Is this OK, how long do I leave him?