No idea.
The only numbering issue I've had is when it reaches the end and starts again so pictures I took recently appear before pictures I took a while back when pictures are ordered by name.
Is there an option in the menu relating to how files are named? If there is could it have been altered or just altered itself? Maybe it's worth a few minutes wading through the menu just to make sure something like that hasn't happened.
I rename on import to Lightroom, to include the camera and year - this keeps things from my 3 cameras nicely separate and ordered (I also copy to a folder by date, just to be 100% sure!)