I found this answer at the Apple Support site and it did work for me. If you don't follow it exactly, it won't work. I hope this helps at least some people who may be looking for a fix. The below information is a direct cut and paste from the post:
The suggestion was to:
click edit in iTunes
click Parental Controls
Under Disable, check the boxes for "iTunes" and "Allow access to iTunes U". (NOTE: It is important to check BOTH of these boxes. When I only checked the box for "iTunes" but not box for "Allow access to iTunes U", this procedure did not help me.)
This opened up iTunes U, though I could not log into my account or get into iTunes Store.
Then I went back to Parental Controls and under Disable, I unchecked the box for iTunes.
When I did this, the iTunes store opened for me.
I live alone. I've never used the parental controls on iTunes. I don't know why I was locked out of the iTunes store in the middle of a session and I don't know why following the procedure above let me back in.
I hope this info is helpful to others.