If you update your computer to Flash Player 11 it can cause problems in Frontierville, specifically with vanishing buildings etc.
Until reasons are discovered as to why this is happening there IS a temporary fix.
First, download one of these files and install, this is an old version of a WORKING Flash Player.
32-Bit Windows:
http://www.oldapps.com/flash_player.php?old_flash_player=7361
64-Bit Windows:
http://www.oldapps.com/flash_player.php?old_flash_player=7360
This should hopefully fix Flash in all browsers except Chrome.
To fix Chome follow these instructions but DISABLE THE
NEWER VERSION! Yes, the NEWER version (ill end in 63).
http://gameplay.frontiervilleexpress.co.uk/2011/09/adobe-flash-issues.html
That is a proven way of fixing the issue until a better, permanent, solution is found.