Scary moment at the Gin Blossoms concert in New York this weekend ... when the bass player COLLAPSED right on the stage ... and the whole thing was caught on tape.

The 90s band was playing a gig at The Paramount Friday night -- and right in the middle of their smash hit "Hey Jealousy" ... Bill Leen stumbled backwards and crashed to the floor, appearing to pass out.

In the video, you can see a stagehand race over to help out while Bill is sprawled out on the floor.

Bill was dazed and confused when he hit the ground ... but we're told he was back to normal moments later and refused further medical attention.  

A rep for the band tells us ... Bill's condition was caused by "exhaustion from the road" ... and after a good night's sleep, he was well enough to perform the very next day.