This happened in Vancouver on the '75 'Welcome To My Nightmare' tour.
[He] slipped and fell into the pit--wonder if he had a few that night--tried to do a few more songs--passed out--hospital--had to postphone a couple of shows. I have some wacky pictures from then including right after trying to continue the show with his head wrapped in a bandage, the ambulance pulling away from the venue, and recuperating in his hotel room surrounded by flowers and get well wishes.
(Renfield, July 1995)
Rolling Stone - 13th July 1975
The status of Alice Cooper's tour was thrown into doubt after AC fell off the stage during his June 23rd ('75) Vancouver show, breaking six ribs and suffering a concussion. The accident occurred after the opening number, "Welcome To My Nightmare", as AC chased the Nightmares into a ten-foot-tall toy box. The box tipped over, flipping Alice on his head into the security barriers.
After being treated in the dressing room, AC returned to the stage wrapped in bandages. He was able to perform several numbers before ending the abbreviated show to cheers from the 17,000 in attendance. He was then driven to the local hospital, where he received 15 stitches to his head.
AC appeared against his doctor's advice at his next date, in Edmonton, three days later, although warning the crowd he wasn't fit enough to put on an entire show. 35 minutes into the performance he collapsed after experiencing breathing trouble. Ticket refunds were offered, although no one was reported to have asked for one.
The next day, manager Shep Gordon sounded a pessimistic note on the status of AC's next show, in Minneapolis June 28th:
"Everybody around him says he looks horrible.
But on the day of the performance, a publicity aide to Cooper said he was feeling "pretty spirited" and that the show would go on, adding "he's just going to be really careful."