This happened to us too, my daughters been using her toilet for almost a year and about a month ago she started wetting herself and asking for nappies to go toilet in. I basically looked out for when she needed to go toilet and we went together at the same time, which really helped her get back into it!
We had a regression especially at nursery. We just kinda kept going and just reminded him more and got some sticker chart and it passed but it wasn’t like fully potty training again.