Windows Not Responding

I have 5 pages opened on my screen to work for the day.

One for my emails and Reference ... the windows Hotmail and Wikipedia.
Another for my favourite 'forum' to post entries.
One Note Pad ... Untitled to note something of value.
Another Note Pad that has 'saved' notes.
And, Word Web 'thesaurus' for reference.

We happily plod on going from one to the other.
Suddenly one of them ... windows mostly ... stops work ... and the words in bracket comes 'Not Responding' . . . Internet connection is ok with that 'green' round button at right hand bottom of frame with word 'Internet'.

What does this mean ? Is there any remedy for this ?

Please help me.