Cubase Essential 5 Cannot find HALion One plugin

I am running Windows 7 with Cubase 5 Essential and have been for quite some time.
The program has been working fine up until today when I tried to work on a song that I had used the HALion One on. When I opened the .cpr file it says it cannot find the HALion One plugin.
I have also tried to open a new template using the HALion One and the same thing happens.
I have uninstalled and reinstalled HALion One and uninstalled and reinstalled Cubase Essential 5 as well but I am still having the same problem (cannot find HALion One plugin).
Can anyone shed some light on this for me?