The hypnotist later wrote on his Facebook page: "I would just like to let my fans know that I am completely fine.
"A little bruised, but that's all. Thanks for your support tonight, it was a great show with some great volunteers."
However, Alan Coman, treasurer of the Royal Manor Theatre, has disputed Mr Days' claims that he passed out and said the episode was "only a joke".
"It was part of a project for students who were filming the whole thing... but they (the people on stage being hypnotised) weren't pretending because they didn't get up to help.
"The audience didn't know (that it was part of the act) but it was purely to test hypnotism," he said.
But Mr Days' manager, Tara Nix, insisted it was not part of the show.
An audience member talked to BBC about the show, insisting that it initially seemed to be a joke, but when Days was pulled backstage and someone ran to find a first-aid kit, they realized it wasn't.
"At first the audience, including us, found it very funny and thought it was part of the act, but as time went on we began to realise that it was not part of the show and he had actually hurt himself.
"At this point we become very worried not only for David Days but also the guests that were onstage oblivious to anything as they were still hypnotised.
"They simply just sat there 'asleep'."
Though nothing like this has ever been reported, Days does keep a back-up hypnotist and a recording on-hand to bring volunteers out of hypnosis in case of emergency.