When I want to search something in the documentation using the search bar on the top right or using doc in the command window the new window that usually opens doesn't open anymore. If you hover over the matlab icon on your task bar it does show an empty help window, but when clicking on it nothing shows up. I have matlab 2018a and windows 10.
Use the "recent versions" of the operating system or which backup you use.
If you did not create a backup of this folder - bad. But not a drama. You can re-create it easily: Stop Matlab, rename the preferences folder and restart Matlab. The preferences are re-created with default values. You have to setup your preferences again or you can try to move some of the files of the original folder to the freshly created one until you identify, which of the files let the help window disappear.
If it works and you have setup Matlab to your needs again: Create a backup of the preferences folder.
For all readers who do not have problems currently: Perform the last advice:
CREATE A BACKUP OF MATLAB'S PREFERENCES FOLDER
In your case it was the windows position of minimized state, which was saved to the preferences files. Of course using the "Maximize" function in the Windows selection is an easier solution. But a backup cannot be a disadvantage also: Answers: Backup of preferences folder