Why doesn't an application respond?
It may be due to several reasons. It is most likely to be due to a lack of resources dedicated to a specific application. This can be resolved by adding more RAM (Random Access Memory) to your computer, changing the process priority or closing unneeded programs.
It may also be due to a hardware conflict in the harddrive.
Solution
Firstly, identify the application via noting the application window name (ie. Internet Explorer)
or the process name (ie. iexplore.exe).
Launch task manager by doing Ctrl+Alt+Del
Either,
Click on the window name under Applications tab and click end task
Or,
Select processes tab, locate the process name and click end process.
Doing so will result in loss of unsaved data.