It doesn't take long working with metal clay to discover how fragile the clay can be in it's green-ware (dried / non-fired) stage. No matter how many times we hear "handle gently", inevitably you will encounter breakage.
To repair a broken piece, simply follow the following steps.
With your newly found definition of GENTLY, position the two pieces close to each other. Ensure that the two pieces are aligned. Don't worry if you are missing some small pieces, you can fill the spaces once the pieces are re-attached.
With your small paint brush, moisten the edges to be attached liberally with distilled water.
Use “slip” (paste) to reconnect your piece by painting each edge with the slip. It is better to have a little excess slip rather than any gaps or uncovered areas. If you do not cover the entire length with slip, you risk creating an air pocket = very, very bad! Any spaces or air pockets in your clay can have a severe (non-repairable) impact on your firing or a fragile creation that will be susceptible to breakage at the least.
Re-attach the pieces, gently (that's the new gently, remember?) but firmly press them together expelling all the excess slip. Wipe away any excess with a makeup sponge (dampened in distilled water).
Let the piece dry naturally for 2-3 hours. You can use a hair dryer to speed up the drying, personally I do not do this (read this to know why). I find that with natural drying, you can see if the join needs a little extra slip should you see any spaces or cracks. Using a hair dryer can dry your slip much faster; this means that you may still have a crack or space that is not visible covered by a very thin layer of slip.
If you where missing small pieces, you can now use the same steps adding slip - let dry - sand with 600 grit sandpaper and then add more slip and repeat if required.
It can be very frustrating or discouraging when a creation breaks, but knowing how to repair it will help reduce this (maybe not always, especially if you are close to completion). But keep in mind, the very best learning's come from these types of experiences.
So great job, your breakage taught you a lesson that will last a lifetime....until the next time.