I had my memory stick left in my notepad while I was working, when I got a message saying that there was an unrecognised USB, I took the USB stick out and reinserted it which didn't work, and the computer couldn't find it when I clicked on the icon either. Tried to find it on my PC as well, with no luck. So basically my USB stick seems to have just stopped working...any ideas??

try right-clicking on my computer, and going to manage. down the left you will see an option; Device Manager. click on this (with your memory stick plugged in) and you will get a list of devices. try and find your memory stick, right click on it and press format. after this it should be usable again.
Alternatively, if your mem stick doesn't show up there, there is also an option of "Removable Storage" again, try and locate your mem stick, and see if you can format it. good luck

Bear in mind that, if you do that, it will wipe the contents of your memory stick so you won't be able to get any existing data from it. Have you tried the memory stick in another PC? You might find that the memory stick is actually OK but that it is the USB port on the notepad that has packed up.