Can't scan to PC with Brother MFC 8840D

My guess is a driver problem. I'm assuming you can print to it, so you know the USB connection is good. And you can make a copy (I assume), so you know the scanner mechanism and its connection with the rest of the machine works.
The scanner driver is separate from the printer driver. It's entirely possible to install and use the printer without having loaded the scanner driver. Best bet is to unplug the unit from your computer, then uninstall it completely from Windows. There may be an Uninstall program listed under Brother in your Start menu...All Programs... Or go to Start...Settings...Control Panel (or however your version of Windows get there) and open your Scanners and Cameras folder. If you find the Brother machine in there, Select it and click Remove. Do the same in the Printers folder.
Don't plug your machine back in until you have reinstalled the drivers. Go to www.brother-usa.com, and in the Service & Support box on the right, click Downloads. That will take you to a page where you'll select Multi-Function as your Product Group, then the model.
You'll get two lists of links. Forget Manuals, look at Drivers on the right.
Under Type of Software, click ALL. Select your OS.
You'll get a list of drivers and utilities for your model. Select the Full Package. Brother makes you accept the Terms of Use, then starts the download.
Look for Setup.exe in the main directory, or you may need to browse folders labeled with different language abbreviations to find it.
Start the installation, and don't plug your USB cable in until the computer tells you to. it will install the printer first, then return to install the scanner. It may require you to restart Windows more than once. Sorry.
Once you've suffered through all that it should be fine.