Unfortunately, no one here is psychic, so a little more information would help
Seriously though, is this an old .capx? What version of C2 are you using? Can you open other .capx?
Is it feasible to upload the offending .capx so we can try to open it?
Try to give as much info as you can, such as OS, exact error message, does the .capx use non-standard plugins, etc.