I had the same exact problem just now. (on BlueHost though, not hostmonster) I just did some googling and this solved the problem…
First, I checked a php info file, and saw the build date for PHP was just a couple hours ago, so I’m pretty sure it was something that changed on the host’s end causing the problem.
Here was the fix I found…
In the main Magento directory, there’s a file called php.ini.sample
Rename this to just php.ini, and add the following lines to the bottom of the file.
That worked for me and both the front end and admin panel are working again as they should.
A much more detailed explanation is here: http://www.magentocommerce.com/boards/viewthread/16158/