When the shrub begins to grow again in the Spring 2009, it will throw out new stems from the crown. Pick the one stem that seems best suited (should grow straight up) for the job. If you have stakes, consider using them to tweak it if it tries to grow slightly sideways (leave the stakes several 2-3 years. Remove/prune off all the other stems and concentrate on developing this one stem. If for some reason you cannot find a good stem this season, defer the choosing of a stem for next season 2010. To force the stem to branch and increase the spread of the shrub, pinch or prune off the top growth. It takes several years but in the future, you just need to "maintenance prune" (prune to maintain the shape).

I'm afraid I'm not going to be as optimistic as Luis about your success in getting this grow back into a tree form :-( There is no species of hydrangea that is a "tree". Hydrangeas that are trained as standards (tree-form) are done so from a very early age - they are just hydrangea shrubs that are grown as a single, staked trunk. Since yours is a mature plant, regardless of what's happened to its top growth, it is unlikely to produce a strong enough upward growing stem that can be supported and encouraged into a single trunk, rather just a lot of floppy, low growing suckering branches.

You can certainly make the attempt - no harm in trying - but be aware that it may not be very successful. You may just want to purchase a new, very young plant you train in this method or purchase one that has already been trained into a tree form. If you try on your existing hydrangea, follow Luis' advice but definitely provide a stake or other secure vertical support and tie the stem off at several points along its length. And cut off any other basal suckers that may appear, as well as any laterals off the selected stem except for those at the end. It takes about 5 years for this process before the single stem has developed enough girth to be considered a free standing "tree". That's one of the reasons tree-form hydrangeas are expensive to purchase.