My monoprice printer was working; I then tried to format the SD card on a Mac, then it stopped working. I have now tried three different SD cards after formatting on a Windows PC, and still nothing works. Are there supposed to be files on the SD card itself like some kind of root files?
I've no idea why 3D printers don't like SD cards formatted by operating system utilities, but they don't. Use SD Memory Card Formatter to format your memory cards. Unfortunately, you have probably lost any data that was on the original SD card.
The SD cards that are supplied with 3D printers typically contain documentation, test files, and device drivers. However, these files do not need to be present for the correct operation of the printer. All that is needed is a correctly-formatted SD card containing the G-code files that you want to print.