Hello Mel,
I would first suggest checking whether the mouse pointer can still be moved around the screen, and, if so, whether clicking on stops changes their states as expected. It may be a hardware fault (e.g. video card), or a driver fault, or maybe you've just loaded too much data into the available memory (e.g. because a RAM board has recently failed) resulting in the computer performing extremely sluggishly.
Let us know the answers to these questions and hopefully you can determine where the issue is.
Thanks.