Slightly different situation for me but my 5yo daughter has just started weeing in her pants during the day after being trained for 2 yrs.
We have had much success after making her go to the loo every two hours without fail. We think she's just forgetting, or gets so caught up playing that going to the loo is a major bother and interruption, so she just doesn't do it unless she's busting, and by that time it's too late.
Hard not to get upset by it, esp if it has been fine for so long then all of a sudden backwards progress without explanation. But we just try to ignore the wees in the pants, we just say "oh OK, pants are wet, let's get changed" and then praise praise when pants are dry.
Good luck.