I have a single handle Delta tub faucet w/o a shower. The faucet diverter for the non-existent shower has become engaged, the ring at the spout exit. With the pressure diverted to the non-existent shower, how do I get the pressure back to the spout? I've tried messing with the ring but to no avail with the handle on "off" & "on"...

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You should have disable the diverter so this cannot happen...

What kind/type of diverter is it ? If the knob is at the tip of the spout, try to insert skinny flat screwdriver bet. The spout and the diverter ring. Just prey it apart a bit. Once air gets in, it will get loose. Or try to take it apart by unscrewing the diverter know and pushing the threaded rod downward.