In that case.. It’s going to be module related. Something isn’t loading correctly.
Create a directory called DISABLED
So that it is app/etc/DISABLED
Move any custom module .xml files that you have from
app/etc/modules into the DISABLED directory
Check the app/code/local directory
If there is a directory called Mage, rename it ~Mage
Make sure that you disable cache / compilation
Refresh order view page and see if it comes back to life.
If it does come back correctly, move each module.xml file back into the modules directory. After each move back, refresh the order page and see if it goes again and you will find the module that is the problem.
Alternatively, turn on logs and check
var/log and see if you have an exception.log or system.log file
Or check your php error_log.