IF using Internet Explorer has problem for check out.
It sounds like there is some cookie
setting in your IE browser that is not set correctly.
This is not normally necessary since the default settings for Internet Explorer are known to work with the cart but verify your cookies are set as follows in Internet Explorer:
1. Click on the Tools menu and then Internet Options.
2. Click Privacy and then click Advanced.
3. Click Override default settings, and then instruct Internet Explorer to accept cookies from first-party and third-party websites.