Photo file numbers in nikon d3100 all mixed up?

When I first started taking photos the automatic photo name was DSC_0001, DSC_0002 etc. But now the photo name has changed to _DSC001, _DSC002, etc. So they are all mixed up and out of order when I download them to my computer. I'm not sure if something happened to the settings in the camera that changed the file name, but can someone tell me how I can get it back to being DSC_(then the number), Not _DSC(then the number)

You need to make sure that you have the File Number Sequence option in the ON position, Page 145 in your user manual

The colour workspace can be either sRGB or AdobeRGB. Page 65 and 133

Files shot in sRGB are numbered thusly. DSC_1234
Files shot in AdobeRGB are numbered like this, _DSC1234.

same thing happened to me where my file numbers kept resetting to 0001. Not sure if this video helps but it helped me so this way I continue with the number I left off.