1) It’s hitting some aborting error, and your webserver is configured to be paranoid and to not say anything in those situations (which is a good thing). In those cases, look at your webserver logs (if you’re on a cPanel/Plesk site, you can get to them from your control panel; otherwise use locate to find httpd.log or apache.log most commonly) to get an idea of what’s wrong.

1a) Common causes are missing modules such as gd or mysql, wrong directory permissions, etc.

2) You’re running out of RAM. This isn’t so common with word press; WP isn’t an insanely hungry app. Even so, just try pumping your allocation limit up (remember to restart your webserver to make the change take effect) to see if it changes things.

3) One of your plugins or your theme don’t like the new Word Press. Briefly, turn all your plugins off, and set your theme back to default, just to test. (If the control panel also won’t come up, it’s probably not this, but still testable: add a ‘Z’ onto the end of the wp-plugins and wp-themes’ directories’ names. It won’t break anything, though it’ll look awful briefly.)

4) You have a partially uploaded file. This most commonly happens when you’re trying to work over modest quality wireless, but some shared webhosts so badly overburden their servers (cough M2O cough) that FTP connections are occasionally just dropped. If neither #1, #2 nor #3 turn up success, try re-uploading everything, just for the sake of it.

If none of those four help, c’mon back, and we’ll keep going from there.

<?php phpinfo(); ?>

The PHP error log should tell you why it’s aborting. Common causes include that PHP believes the directory your blog is in to be off limits, that some security setting like including other files is being violated, that an extension PHP is trying to load is corrupted, et cetera.

Are other PHP applications working on this site? If you don’t have any, just type this into foo.php in the same directory as your blog, then open http://yourblog/foo.php :

If you see a big purple table of semi-cryptic settings, you got the right thing, and PHP is otherwise working.

Let me know how that plays out and I’ll try to help you out.

Generally, it’s because one half-uploaded file stops in the middle, and PHP kills itself when it realizes there’s missing code. Since WordPress uses code as themes, to keep things flexible, this means that themes can cause incomplete code halts.