You will probably want to replace the string if it has broken. Chances are that it is worn enough that it will break again even if you can get the string hooked back together. Here is a place that sells the string:
RV Shade and Blind Repair
They have some instructions on their site, and they include an instructional CD with their repair kits. If you do take yours apart pay very strict attention to where all the string goes.
Also, if the buttons that you tie to are broken and need replaced, I have seen where people bought metal sewing machine bobbins. They work better than the plastic, in that, they don't break.
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