I made some modelling paste and left it to sit overnight as per the recommendation. I've never made it before so I followed all the instructions carefully and wrapped it nice and tightly. When I came to use it, it had gone really hard !! I cut a piece off and it is softer in the middle so I was able to knead it but it is really crumbly and I couldn't get it to go soft or pliable again.
What have I done wrong and can I rescue it or should I just throw it out and start again?!

That crap always bugs me. Haven't been able to get it right since school. Yeah, just knead it up real well - thats just the way it is.

How long do I knead it for? I was at it for a good ten minutes and it still had hard bits in it It's like it's formed a shell on the outside and the shell won't reabsorb into the soft stuff when I knead it. Will microwaving it help? I didn't try that.

Kitty, Know what I did when that happened to me? lol
I sat and picked out all the huge hunks then kneaded it adding a little shortening as I went along until it was right. HTH. Been there! It SUCKS

Yea, i would try the microwave - watch tho it won't need very long and will heat up from the inside out.