I have a Lexmark printer that won't print any documents because it say need new ink cartridge. I haven't printed many documents, so I just took out the cartridge, shook it and replaced it. Why am I able to print test pages but not word documents? Is there a reset button?

First of all, don't shake the cartridge!
It is interesting that it will print test pages, but not Word documents. To me, this would indicate an issue with the computer, not the printer. However, since it says it needs a new cartridge, we'll go with that.

Is this a genuine Lexmark cartridge, or a refilled cartridge? The refills sometimes cause a lot of problems. If it is refilled, I'd suggest getting a new, genuine Lexmark cartridge for best results.

How old is this cartridge? You say you haven't printed many documents, but if the cartridge is old, they will dry out over time. If it is printing test pages okay, it sounds like it is not dried out.

That being said, I will tell you that calculating the remaining ink level in a cartridge is very difficult, and an inexact science. There is no mechanism like in your car's gas tank to actually measure the ink level, so instead all inkjet printers rely on counting the number of dots that are printed to indicate the remaining ink. Many factors can affect this calculation, so it is at best a "scientific guess".

Now, if your cartridge is genuine, and not dried up, and hardly used but still indicates empty, you might try going into the printer menus and telling the printer that you installed a new cartridge (i.e LIE to it) and see if that makes the low-ink warning go away. For sure, your indicated ink level will be wrong, but just keep printing until the quality starts to suffer and then get a new cartridge at that time.

Best of luck,
WallyH