It has suddenly become impossible

There are many reasons for sync problem errors between cameras and computers, making it very difficult to troubleshoot here. Would first recommend that you try plugging the USB cord into a different USB port on your computer. If that doesn't work, would highly recommend that you consider using a card reader instead to download you pictures. Card readers are much more faster in downloading pics, do not require the use of your camera's batteries, and provide much more reliable downloading of the pictures (fewer errors). They're also VERY inexpensive these days. Here are some examples from Amazon:

http://www.amazon.com/s/ref=sr_st?keywords=card+reader&rs=172282&page=1&rh=n%3A172282%2Ck%3Acard+reader&sort=price