My usb flash disk not seen to my computer

Hello,
I have experienced suddenly that my usb flash disk not recognisable with my laptop computer. at the begining it was alright, that is, I was able to open it and read the content but nowadays I can't do that. so please would you help me to find the solution because I have so important data in it. for the information I am using Window vista home premium with kespersky ant virus.