Most likely, you saved it in Photoshop with menu File - Save for Web. Which is fine, it is still the same image, but that "Save for Web" purpose is to remove all of the Exif data, which is where the JPG dpi number is, so it disappears. That saves a few bytes, and video monitors have no use for the dpi number anyway.
Menu File, Save As (JPG) will Not remove the Exif and dpi info.
Then when Windows programs try to show the missing dpi number, many of them call it 96 dpi, or possibly 120 dpi if Large Fonts was selected in Windows (this dpi number has no meaning to the image pixels). Photoshop itself shows missing dpi numbers as 72 dpi (which has no meaning).
The only significance of the file dpi number is that when you print it at home, the default option is to scale the printed size on paper according to the dpi number (3000 pixels at 300 dpi will come out 3000/300 = 10 inches size on paper).
When printing in Windows, you can simply change dpi number then too, but pay attention to it.
But if you send it out for printing with instruction to print 8x10 inches, it will be the 8x10 inches regardless, ignoring the dpi number (so doesn't matter what the dpi number is then). Size in pixels is all-important though.
You can easily use the Photoshop menu Image - Image Size to make the dpi number be anything you want. To do that, uncheck the Resample Image check box (bottom of that Image Size box), and then it will not resample, and only the dpi number will change, so that no pixels will be affected. The dpi number only affects future printed size. Dpi in the file is "just a stored number", it only affects printing (if the printing choice looks at dpi).
But if you instead Resample, then both image size in pixels (and the dpi number) will change, possibly drastically, which if not intended, can do much harm to the image size.