How recent is your last db backup?
What do your error logs say? Can you track down a specific error causing the blank page?
Replacing php files isn’t going to help you much. You are going to need to try to undo the changes that caused the problem, and that’s going to be in the database.
I haven’t encountered an error like you describe, but I have backed magento out of a few nasty corners and I know that quite often you are going to have to do your repairs in the phpMyAdmin to the database after finding if there is a specific php error causing the issue.
Depending on how frustrated you have gotten and how many other things you have changed because of it, just resetting the default admin page to restore the former working state isn’t too difficult - look in the table core_config_data, and find admin/startup/page in the field: path. Try editting the field: value back to dashboard. If that doesn’t restore it, we are going to need to do something more drastic.
I am hoping you have some experience with phpMyAdmin (and that your server has phpMyAdmin installed...) in order to avoid having to give you any step by step help operating this db repair.
If things go badly, expect to have to restore from a backup. You DO MAKE BACKUPS, right?
Just so you know, on my localhost install on a Vista machine using Magento 1.3.0 I switched the admin start up page to Sales->Orders, logged out and logged back in and saw what was expected - the sales/orders page. Are the admin ROLES set to allow your other admins to see the Sales/Orders page?
After that, just for S&Gs;I went into phpMyAdmin and found the field for the admin startup page as per the instructions I provided above, where it now reads “sales/order” and switched it back to “dashboard” and there you have it -> startup at Dashboard restored from phpMyAdmin.