Your wp e-commerce plugin flaked out (If you have access to your admin area, just log in and delete the plugin)
If not, If your host has a file manager (most do) where you can log on to manipulate files, you will need to go there and go to the:
wp-content folder, then the plugins folder -- delete or rename the wp-e-commernce plugin to let yourself back in
If your host does not have a file manager, you will need to learn about using FTP to do the same