Lucy, you might try the usual easy "fix," - log out, clear your cache, re-start your browser, even re-start your computer.
Sometimes an issue similar to this is caused by a corrupt cookie. Rather than deleting all cookies, which can be an annoyance if you have a lot of saved info, you can delete just the cookies related to ancestry.com.
The instructions for deleting all ancestry cookies will vary depending on what browser you are using.
You might first try viewing your tree with a different browser, to see if the problem is still there.
I tried with 4 different browsers, and cannot get the view MB displayed.