The online retailer Amazon.com said its United States Web site was down for about two hours on Friday but said it was unsure what had blocked access. An Amazon spokeswoman said the site was unavailable for a couple of hours starting at around 10:25 a.m. California time. The Web monitoring company Keynote put the outage at two-and-a-half hours and said the site was fully back up just before 1 p.m. in California.