How to turn on debug checkpoints in my source code

[GPerk]

When I started working on a program today, suddenly the debug checkpoints (Red dot on left by linenumbers) DONT'T WORK.

I am trying to find the cause of "Argument out of range exception was unhandled" error, but without the checkpoint I can't find the cause.
How can I turn the debug checkpoints back on?

 

[GPerk]

More info on the post:

When I set a breakpoint, the icon on the line is bright red.

But when I run the program, these icons turn to a light color with a tiny ! in the lower right edge.

It appears that I have breakpoints disabled.
How do I turn them back on?