Hi pitchounette
I am having the similar problems with dd1 who is 3.2yo. She was fully potty trained and going by herself without any prompting or even mentioning it to me.
Then suddenly she was having accidents all the time and never being able to hold it in. Combined with a complete refusal to ever sit on the toilet/potty, with tantrums every time.
DH was absolutely furious with her, and started instilling Victorian style discipline (when he was around), which didn't work and made the whole house a nightmare for us all.
In the end, I took her favourite slippers away until she could stay dry for a day, and introduced chocolate buttons for every wee, with two for a poo (yes that is terrible, I have never resorted to bribery before, but was at the end of my tether).
This was combined with putting her on the toilet every hour, and trying to get her excited about the chocolate buttons. "I wonder if there will be a wee and then we can have a chocolate button".
Eventually, two weeks later, she is starting to get there. We have had no accidents for a couple of days, and she is much less reluctant to sit on the toilet/potty. It is still a pain, but I can see that it is going to get there.
Every child is different, but I hope if nothing else, you will realise you are not alone. This is so common - I have posted on at least 3 different threads over the past 3 weeks!