Disappearing variables

Hi.

I have 4 variables that I setup with an A in front, so they would be at the top of my variables list as I use them a lot eg AProgress.

All these variables are numeric, my only other A variable (AudioON) is a True/False and it has not disappeared.

I set these "A" variables to different values throughout the project, once the project is finalised.

Initially AProgress is set to 0, until I finish development then I manually assign different values to it throughout the project.

Up until today, I have not had a problem with them.  But now all my "A" variables have disappeared and instead I am left with "Set unassigned to unassigned" throughout my project.  

Any thoughts?

Yes I have rebooted, etc.

Yes I have checked another completed project and the variables are still there (thank goodness as I would not want to have to reassign values to them again).