Pop window opens that requires me to type in my username and password

I am having a problem with a theme that I developed a long time ago. I developed and tested the theme with WordPress 2.3.1 and it was functioning alright. Unfortunately my client was not ready when the project should have been completed and the project was put on hold. There seems to be a problem with the theme and with WordPress 3.3. When I try to view the blog a pop window opens that says “To view this page, you must log in” and it requires me to type in my username and password. I never had this problem before. I’ve been trying to figure this out and it’s driving me crazy. I would really appreciate it if someone would let me know what is causing the problem.