ImageCache not working
I found a solution to my problem. There were two changes I needed to make. 

The first was in the .htaccess file in the "files" folder there was the line:

SetHandler Drupal_Security_Do_Not_Remove_See_SA_2006_006


When I commented out this line I could see the images again. 

However, I was still having problems generating new ImageCache images. I managed to fix this by adding the line below to the settings.php file:

putenv("MAGICK_THREAD_LIMIT=1"); 


I find out this last solution from my hosts (1&amp;1) technical support.