You've recently upgraded to MODX Revolution 2.5 or above from a version below 2.5 and now get a blank page or 500 error when you try to edit a resource.
If it's only Resources you cannot edit but you can edit/access other parts of the MODX Manager, it's most likely that you need to upgrade TinyMCE.
Older versions of TinyMCE contain a bug that was brought to light with changes in MODX Revolution 2.5 and therefore should be updated. You can update TinyMCE by clicking Extras>Installer and locating TinyMCE in the list of installed Extras in your site. You should see a highlighted Update button.
It's good practice to always check for Extras updates after you upgrade MODX.