There are so many users who are experiencing this problem. The problem was mainly determined to come from the ISP that the users/customers use:

It appears that ISPs like AOL, Comcast, and Shaw.ca use a technique of changing IP addresses that cause security settings in Magento to empty the cart and not allow people to check out.

It turns out the fix is fairly simple. Change the Session Validation settings in the Magento Admin, found under System > Configurations > Web,  to ‘no’ on everything except “Validate HTTP_USER_AGENT.” After doing this,  go to System > Cache Management and refresh the configuration cache to apply the changes.

By applying these fixes to our site, people who could not place orders were suddenly able to do so. When you consider that, perhaps, one person in ten might actually call to complain, I shudder to think of how many sales might have been lost before we applied the fix.

[1] http://thewellrunsite.com/2009/02/12/fixing-the-magento-checkout-glitch/