Clear Internet cache in Internet Explorer (IE) click here.
If no joy with above, could be a cookie problem - so make sure you have IE to accept cookies. IE>Tools>Internet Options>Privacy>Advanced. Put a tick for "Override automatic...". Select 'Accept' for First-party Cookies, 'Block' for Third-party Cookies & put a tick for "Always allow session Cookies". Press OK, OK & try again.
If you're still having problems, try another browser like Firefox click here or Opera click here (both free).
As a last resort, try ordering from another/friend's PC, or try another day as the site could be having problems. Good luck, G