items can not be added to shopping cart?? HELP!!

three of my customer told me that they can not add items to carts and show a note like below but my page is ok for items adding to my shopping cart.
2006 MySQL server has gone away
in:
[select code, title, symbol_left, symbol_right, decimal_point, thousands_point, decimal_places, value from currencies]


what is the problem? anyone has same question?

This means that you have a problem with the php adding or calling from the the sql database when they click add. Without seeing your page or seeing the issue, or even knowing what platform you're using, this question is basically unanswerable.