Morgan Wallen fans in Mississippi aren't waking up singing his songs, or his praises ... because he pulled the plug on a concert just minutes before he was set to get onstage.

The country singer was about to jump into night 2 of his shows Sunday night at the University of Mississippi and the crowd was ready to hear some hits before screens around the stadium displayed the last thing they wanted to see.

The message told concert-goers that Morgan's voice was shot and he wouldn't be able to perform, adding they'd get refunded ... but that didn't seem to help, because people obviously were still pissed -- sharing some outrage on social media after they were left high and dry.

Morgan took to Instagram after the last-minute cancelation ... telling fans he spent the day resting up after his voice started to go -- adding "I really thought I'd be able to take the stage and it kills me to deliver this so close to showtime."

That didn't seem to make much difference to upset fans, especially some young kids in the crowd who left in tears.

As we reported, Morgan had some more trouble at his Louisville show Thursday ... taking a hard fall after all the fog onstage made it practically impossible to see where to step. Some people in the audience also said there were technical issues during that show, too.

Clearly, some bad juju surrounding Morgan's concerts in the last week. Maybe a reset's exactly what the doctor ordered!