A family of Microsoft word processing software products for creating web, email, and print documents.
If this just started happening where you cannot select multiple words via the method(s) you previously used... then I suggest you reboot the computer. It sounds to me like something has corrupted in the computer's memory.
Also make sure you fully update the software. Go to Word > Help > Check for Updates. And as the last preventative measure run Disk Utility.app (located in Applications/Utilities) and Repair Permissions on your primary drive.
Hope this helps