Mostly unable to access admin menu, Trouble viewing blog

When I went to my blog tonight, I discovered that JetPack / WP Stats was displaying some sections properly, but in others was showing this:

fatal error: Call to undefined function define() in /wp-admin/admin-ajax.php on line 16 (line numbers varying).

Sometimes I can view themes, plugins, etc, and sometimes I get this:
Fatal error: Call to undefined function defined() in ....../wp-admin/admin.php on line 14

(I’m not saying that this is a JetPack issue, just that it’s through JP that I first noticed the issue.)

But I cannot activate a new theme, or deactivate/activate plugins through admin, nor can I edit any plugin settings. My real blog is using the theme Atahualpa, but my test blog is running Twenty Ten, so I don’t think it’s a theme issue.

I get that same error message (with line 14) on an initial view of my blog, but 2-3 refreshes will bring the blog up (and I had someone outside my house try this too, with the same result).

This is my blog: https://glutenfreeandtastyblog.com

I’ve also set-up https://glutenfreeandtastyblog.com/testblog/ to allow me to test WP udpates and plugins without worrying about dumping my real blog offline. It’s having the same issues.

Since I can’t disable my plugins through admin menus, I disabled them through Control Panel & phpMyAdmin — so far just in TestBlog. It didn’t help. Still receiving the same error (line 14).

I am hesitant to disable them in my primary blog since it did nothing for TestBlog.

I cannot post. I’ve recently received product samples and need to be able to post my review sooner than later.

I’ve posted successfully to my blog since whenever the 3.4.2 update to WP came out, and haven’t had any problems. I actually haven’t updated anything, plugin or otherwise, on either blog, since posting on the 17th.

The last thing I added was a plugin called IntenseDebate — but I added it before I made the post on the 17th.. and I only have IntenseDebate on my GF blog, but neither blog works.

I’m really stumped and would appreciate your insight.

Please note, however, that if your suggestions require going through phpMyAdmin (or something similar), please write as if you were writing “phpMyAdmin For Dummies”! I only touch it when I need to, so it’s not a strong point. I will not take offense, I do not want to screw up my blog! ??

Thank you!