Done well, this can work -- but getting it to work well is tricky (as I'm finding.) IMHO, one of the trickiest aspects is to provide a plausible reason that both big changes (tech dies; magic appears) happen at the same time. Why did those both happen? Did something science-y/technical in a lab somewhere cause a world-wide (or universe-wide?) change? Have the stars finally aligned just right to awaken Lovecraftian Old Ones?
“Extraordinary claims require extraordinary evidence.” (--Carl Sagan.)
In fiction, we have more wiggle room than that, but your readers will want an explanation that makes sense (to them), within the premises of your story. The setup/world design needs care, so your readers will buy into your reason why guns, cellphones and cars (for instance) no longer work.
A previous answerer/commenter mentioned Steven Stirling's Emberverse series (there are two loosely-linked sets of novels.) If you haven't read Dies the Fire, IMHO you're in for a treat! That said, I found his explanation a bit troubling. His unseen "alien space bats" did this to earth, but we don't know anything about why they bothered.
Give the readers a plausible reason for the change, and you've got a great starting canvas to work on. Lots of problems/conflicts both from the old ways no longer working and the new (or even Older, if you will) ways coming back and mucking things up for your characters.
P.S: I've seen a few questions about this basic idea on the worldbuilding stackexchange and they've been allowed if posed narrowly enough that people can answer; come see!