Cardi and ONE MusicFest broke the unfortunate news on Wednesday ... Cardi apologized to the fans in Atlanta but says she's still recovering from a "medical emergency" and is focused on coming back better than ever.

The show will go on and the lineup is still stacked ... Earth, Wind & Fire, Nelly, Method Man, Jill Scott, GloRilla, DJ Drama, BigXThaPlug, Fat Joe and dozens of other well-known artists are still scheduled to hit the stage for what will be the fest's 15th anniversary.

ONE MusicFest encouraged fans to uplift Cardi in prayer and suggested they'll be scrambling for the next 24-48 hours looking for a replacement!!!

It's unclear what caused Cardi to be hospitalized but she's definitely seen better weeks. She went live on IG Monday from a hospital bed -- but didn't specify what was going on -- as she went off on whoever prank-called Child Protective Services to visit her house.

Earlier today, she unloaded the clip at soon-to-be-ex-husband Offset ... admitting her day would brighten up if he got hit by a truck in the middle of the street.