I have a Fiesta apple tree being trained as an espalier. I snapped the main stem, all growth in fact, above the first tier. The young tree is effectively now a T, a step-over.
Will it form new shoots from near the T junction, either from the main stem or somewhere along the side branches, but close enough to the main stem for me to take it up as a new leader?
My fear is that the tree won't ever form new shoots suitable to re-train.
A healthy apple tree will produce a profusion of new shoots in an area where much growth has been removed. This will be even more apparent if you fertilize with a nitrogen rich fertilizer.
In your case, without a picture of the damage, here's what I'd recommend:
Take the stump back to the last lateral branch, with a smooth cut. A shoot from the base of the last lateral(s) is preferred to a shoot from the stump for structural strength, because the weight will be placed almost directly centered under the new shoot, rather than sideways onto the vertical stem.
Watch for new shoots, and rub out all but the three strongest shoots that come out closest to the old stump. This keeps the energy supply per shoot high, while leaving you the final choice of which shoot has the best form and placement.
Remember that for the first few years, the shoot will have a relatively weak anchor to the plant. Do not pull, bend, or otherwise unnecessarily abuse the new shoots.
I would wait until next spring before the buds break to choose the best new shoot. Look for a balance between size, placement, form, and overall health.
Nutrients are most effectively move vertically in a plant, so the competition from the existing laterals shouldn't be a problem.
You can start training the new shoot next year.
In the case that your tree does not flush out this year, it will almost certainly do so next spring, and the previous information is applicable.
I have had apple espaliers successfully shoot out even from a severed lower lateral, so your tree should give you a good show, if you fertilize and water regularly, and keep an eye out for pests and diseases, treating as needed.