Entire project going wacko if i change something in the calendar.module file !??!

If i try to change anything in the calendar.module file, even if i undo the changes, all pages in the project are blank all of a sudden.
I used to be repairable, by replacing the calendar.module file with the original calendar.module file from the download.

However, this is not the case anymore and now i can't open the project at all, i can't login to the admin interface, nor do i see any page in my project.

Does anyone know what to do here or what i have been doing wrong?

the module (calendar) is installed under /sites/all/modules, is enabled and all, i'm working on the latest drupal 5.2 version.

I'd really appreciate some help here.

Thx in advance!