Debugger {} not found

Sirs
I have been working on a project on and off for about a year. I set it up initially to enable me to use the debugger, which I used several times a day.
I broke off for a holiday and to do other projects for about two weeks. I have just returned to this project and when I try to debug I get the following error:
Debugger' {Lot's of HEX stuff}' not found.
I don't know what I have done or how to undo it.
One answer would be to start a new 'clean' project and copy and paste 3000+ lines of code. Is there another way?
I am using the Qt 5.0.1 kit with the 'Debug' Build.
If I try the other builds I get syntax errors in read only application header files!
I searched this forum without success. (I cannot believe no one else has had this problem.)
I am running on Linux
Any help would be much appreciated.

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I don't know what I have done or how to undo it.

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Given your vague description, who knows what you could have done? Moved / updated your Qt distribution? Moved your project? Renamed directories or files that your project depends on?

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One answer would be to start a new 'clean' project and copy and paste 3000+ lines of code.

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Geez, why not reformat your hard drive and start from scratch? That's not a solution, especially if you think you need to cut and paste and not just fix project settings to use the existing files where they are already.

First, make a basic, do-nothing Qt GUI project - a main() that shows a QWidget. If you can compile, build, and debug that, then it is likely you inadvertently changed something your other project depends on. If you can't it's likely you 1) messed up your Qt installation somehow, 2) installed some other package that conflicts with something you were previously using, 3) changed your PATH so the wrong things are found first, or 4) did something else.