Sometimes an application will not start up because a previous process from that application is still running in the background and did not properly exit.
To check if the process is still running,
On the Task bar at the bottom of your screen, click on the right button on your mouse. This will bring up a menu. Choose 'Task Manager', then the 'Processes' tab. You can check to see if the processes are there.
For example, Microsoft Outlook's process is called OUTLOOK.EXE, Adobe Acrobat's process is AcroRd32.exe, Microsoft Word's process is MSWORD.EXE, Microsoft IE's process is IEXPLORE.EXE and Microsoft Powerpoint's process is called POWERPNT.EXE. These are the applications that you may run into most often.
To kill, the process, click on the process name, then select 'End Process' at the bottom of the window. After the process, you may restart the application.