I doubt the actual pruning would kill the rose, especially an established rose in the growing season. While impossible to say for sure, more likely something secondary was introduced during the pruning process that your rose didn't like.

If your canes are still green and plump I would give it some time. This is a hot time of year, your rose might be resting. Do you have a way to share some photos?

About the only thing I know of that would cause sudden death during the growing season is voles or gophers chewing on the roots, certainly not pruning.

With octopus-like large shrubs, you can train them as you said, or you can prune for bushier habit. Prune to 3-4' in spring and then cut back each shoot by 1/3 to 2/3 after blooming. The idea is to get more stems that on average are shorter. Severe pruning just encourages it to make a few more very long shoots from the base.

If the canes are still green, keep it well watered.
I would NOT add fertilizer, just use plain water.
You may want to provide alittle shade for it at the hottest parts of the day. Like, placing a chair over it, etc.
If the canes are brown & dead looking, then it may be gone.
I would also think that pruning did not cause this problem.
But, who knows?

This may be a silly question, but I'm still learning as well.

What if the pruning job took the canes down to the point where chas was left with no leaves in the middle of summer? Could that be reason for the plant's decline?

No, an established plant has energy stored in the roots and canes to drive the production of new green shoots. The potted roses in garden centers were harvested in November, severely pruned and stripped of foliage, placed in cold storage as bare-root plants, then potted up 6 weeks before market time and started into growth.

It would take several consecutive severe prunings or defoliations to use up all the stored energy.

Michael, your answer made me smack myself in the forehead and yell "Doh!".....but what if, as in this case, the cold storage step was replaced with 90 degree heat?

I'm guessing the answer would be that it makes no difference, since the plant would simply not go dormant & begin producing as normal. This begs the question, however, of why we're concerned about defoliation during a normal growing season.

Is it really a big deal, or is it a matter of aesthetics?

Wow, what a humming forum you folks have here. I mainly lurk in the 'Carolina' and 'pond' forums. I didn't think to check back on the same day or in just a few minutes. I appreciate knowing that it wasn't horrible to prune so hard. It is difficult for rose flowers here because they seem to be salted peanuts for the hoards of japanese beatles.
Jim, thanks for the extra info, but I'm afraid it is gone. It had a little pale green on 2 old canes yesterday but that is all but gone today. It has been over 100 for a couple of days and in high 90's for a month but the other plants there appear to be well watered.

If it makes it any better, my Teasing Georgia did the same thing a few years ago. Lived through the winter, grew well, then just keeled. Could never find a reason, no voles, no sudden springs, nothing.

"This begs the question, however, of why we're concerned about defoliation during a normal growing season."

In zone 7 and northward, a couple of defoliations during the growing season can lead to death in the following winter because the plant is depleted of the sugars it uses for antifreeze. The stored energy I spoke of us manufactured by the leaves in sunlight. It's good enough to recover from winter, but it's not unlimited and it will be depleted by repeated defoliation. At best you get reduced growth and bloom as well as, temporarily, a butt-ugly plant.

I had this happen to two of my well established roses, one my very favorite. I pruned them hard in summer and they both died within a month. I have to believe that it had something to do with it since they both seemed healthy. I learned to not prune hard in the summer. Ever learning, aren't we.
Nancy

I prune hard (back to two new growth shoots) in March/April - including an Abraham Derby and several other David Austins - and haven't had a problem with them . You're so right teka2 that we learn what works by doing what doesn't!

My climbers aren't pruned quite as hard (Graham Thomas, Crown Princess Margareta) but are cut back by mid-April at the latest, before any blooms appear. I was advised (when I asked why I needed to prune a rose that was precisely the height I wanted it to be!) that long canes with blooms at the top mean more pruning is needed - but only in early spring.