Play video content EXCLUSIVE TMZ.com

"Bachelor in Paradise" star Justin Reich never saw it coming -- a downhill bump caused him to slam so hard on the ground he blacked out twice with devastating injuries.

Justin -- who appeared on 'BiP' two seasons ago and on Kaitlyn Bristowe's 'Bachelorette' season in 2015 -- tells us he was in his hometown of Chicago 2 weeks ago riding his longboard when, out of nowhere, a bad turn flung him off his board.

Check it out ... Justin says he woke up in an ambulance where, again, he blacked out. The injuries are pretty gnarly -- a collapsed ear canal, concussion and fractured skull. He was in the ICU for 2 days but is scheduled to return next week.

The big worry? Possibly permanent loss of hearing ... and in case you're wondering ... he copped to not wearing a helmet.