Barney was sober for a number of episodes and then he was drunk again. Wha’happen?
He stopped going to meetings and didn’t call his sponsor anymore. It’s the same old story…
Did they deal with it in a specific episode or did he just turn all drunk again one day?
They never devoted an episode to it. He sobered up in “Days of Wine and D’ohses,” stayed on the wagon for a while, and then Lisa found him lying on the pavement drunk in “I’m Spelling as Fast as I Can” (he sings the line "R-E-L-A-P-S-E! That’s what beer has done to me!). I think that re-established him as a drunk and he’s been there since.
He originally started drinking in school when Homer convinced him beer was more important than studying. There was an episode that showed him falling off the wagon after drinking non-alcoholic champagne (astronaut training).
Just to give a timeline, Barney got sober in season 11 and lost his grip the next season. Wikipedia says the show hasn’t been consistent with his sobriety/alcoholism, which I think is intentional.
Magiver, Barney and Homer were shown to have partaken of beer even as young children. There was a flashback scene where Homer crashed his little red wagon into a tree and he and Barney made a pact (with the usual level of Homer sincerity, obviously) to never drink beer again.