The possibilities are endless, but it may just be the USB port. Some ports - particularly on the front - are difficult to get some USB sticks fully inserted into. Try a back port and make sure the stick is fully inserted. Does the USB drive have an LED or anything on to check it is working?
What OS is on your PC? If it is XP it may not say new h/w found, but just bring it up in My computer. If it comes up there, do you get an error when you click on it?
also, so you use a USB hub or card? they can cause conflicts also.