Can you give us a bit of information to work with? You said in your original post that "one of the staff came up to me yesterday and said i should let my hair down for once and have some fun at work ". The fun at work sounds like there may be an established social side to where you work? Are you the office junior, part of a team, or maybe a Team leader? Some times the position you hold can dictate how much "fun" it is safe to indulge in.
What do your colleagues do? I can understand you may not feel comfortable, or knowledgeable enough to join any generalized banter but if they have something more structured, even something as simple joining the office Lotto syndicate could give you something to help in breaking down the barrier you appear to have innocently erected.
The member of staff who approached you may also be a good introduction to the lighter side of office life. If they spoke to you as someone trying to help could you not have a quite word with them and ask their advise?