I'm not sure if this is the best place to post this, but I am at a loss!

I have two websites that are built using different CMS (both PHP) running on the same windows server with IIS. I can view both websites from my home PC without any problem at all, but get the 'White Screen of Death' if I try to view them over our office network. However I CAN access the admin section of both sites.

Up until a month or to ago both sites were doing well in Google, but now neither site can be found in the search engine nor can be verified using Google Webmaster Tools and they return a 500 Internal Server Error. However, both sites are returning data to Google Analytics.

We also have other websites running on the same CMS on the same server that don't have this problem at all.

If you can give me any advice or something to try it would be very much appreciated. Thank you.