'3 Doors Down': Ex-Guitarist Dead at 38

Matt Roberts, the former guitarist from 3 Doors Down has died of an apparent overdose ... TMZ has learned.

Matt's dad Darrell tells TMZ he and Matt were just outside Milwaukee for a charity event where Matt was scheduled to perform. Darrell says Matt had rehearsal until 1 AM Saturday and the two went back to their hotel with an adjoining room.

Darrell says he was woken up around 9 AM by detectives who informed Matt had passed away overnight. Darrell tells us prescription pills were involved ... but he believes the possible OD was accidental.

Matt left 3 Doors Down in 2012 for health issues concerning blood circulation in his body.

Darrell tells us Matt was deeply loved by his friends and family and the loss is devastating.

He was 38 years old.