My son had the same problem on his computer, I was able to fix it by following these steps.
I partially am writing this to remind myself to follow this process when I have the problem, as I googled for and found this thread.
This is a tip for more than just Garden Warfare, I want to mention a few different things to help people troubleshoot these kinds of problems.
Sometimes display settings get garbled for games and cause this issue.
The first step, always, is to try pressing Alt+Enter when you reach the black screen.
For most games these days, it will change the games display mode to a Window, which can sometimes fix the problem, allowing you to go into the video settings in the game and set them to what they should be.
If that doesn't work, we can try to directly fix the settings, or remove the settings files.
Most games by large publishers like EA use some standards setup by Microsoft for their games on windows, so you can try a few different things as a result of this.
The reason the user who was able to create a new user profile in windows to fix this, was affecting a specific aspect of that.
Many games store game settings in one of two places;
On Windows Vista or newer;
C:\Users\Username\Documents\My Games\GameName
C:\Users\Username\Documents\GameName
On systems older than Windows Vista;
C:\Documents and Settings\Username\Documents\My Games\GameName
C:\Documents and Settings\Username\Documents\GameName
When the user created a new file, they removed all their existing settings fixing the problem.
Alternatively, you can look in these folders for files named settings.ini, or profile, and open them with a text editor like WordPad.
Many games will create these automatically, so you can move these files to try to fix them, I don't recommend deleting them as sometimes you could break a game and have to re-install.
What I like to do is select the files I want to move/delete, and right click on one and Send to Compressed Folder (Zip) to store them away safely.
After I store them in the zip, I delete the files.
At this point you can try to start the game again and see if the issue goes away. If it does not, once again try the Alt+enter approach again at this point as it may work now.
If that doesn't work, go back into your settings folder, check if the files were created, if they were not right click on the zip file and click Extract Here.
If the files were created, we can try opening them up and looking for things like
FullScreen
DisplayMode
Resolution
WindowHeight
WindowWidth
And then using settings we know work for things like Resolution, or Height and Width
Unfortuantely DisplayMode and FullScreen are settings that could be represented a number of different ways, so all we can do is guess.
Start with 0 and add 1 until it works enough to use the game settings menu to change things to what you think they should be.
