That happened with my son. He was potty trained for about a year. At around ghe age of 4 he started wetting again day and night. He is over 5 now and still wets every night and has accidents several days. I am tired!!!!! I took him to the dr who sent him to get a Pediatric Ultrasound on his bladder, kidneys, ducts, etc. So far, they haven't found anything wrong. I guess they'll just grow out of it. I do believe they just get busy playing and doing things that they don't want to stop to go the the bathroom. My Pediatrician says to make it extremely inconvenient for him when he wets. Instead of having everything handy to change him the minute he wets, make him take another trip upstairs to his drawers to get clean clothes. Another trip to a different place to get a wipe to wipe off. Then, another trip to throw the dirty clothes in the washroom. She said he'll eventually get so tired of going through this long process, that he'll realize it's easier to just get up and go to the bathroom. So, we'll see...
Good luck!