Thank you guys for your suggestions. The log-in problem is not the main issue now. The CO phone agent can NOT see the flight on a particular day in their system at all, though it shows up on continental.com on that particular day and phone agent could see the flights around that day. What is going on?

Update: I called UA and they found the seat on that particular day without any problem. However, CO phone agents still could not found that flight on that day. They insisit they could do NOTHING until they hear back from their system support people by email, which could take more than a week. Any other suggestions on what to do to expediate the process?