Our site was working fine for months, and now it's not working. We weren't in the midst of any sort of update and hadn't even edited pages in the past few days.
The header displays, but the content and footer do not. When I go to the admin panel, much of it works, but several pages don't:
The Themes page doesn't list the themes.
The Theme Editor page has an Internal Server error.
The Plugin Editor has an Internal Server error.
The Write Page page loads but doesn't have a text box to edit the page.
I tried re-uploading wp-admin and wp-includes with no luck. I also tried editing the posts in the database. And this is the WEIRDEST part -- if I edit the database so that the Page only includes simple text, it is visible. If I add any HTML -- even a simple h1 tag -- it stops working.
Any ideas? This is a live business website so we want it back up ASAP. Thanks in advance.