Can't update WP, can't add new widgets, can't update plugins

The title pretty much says it all. To add more detail: when I try to change existing widgets, they just revert back to whatever was there before.

The automatic update of WP sends me to a page on my site that says what you were searching for couldn’t be found. I wasn’t searching for anything, I’m trying to update WP.

Also my tags don’t appear when I click “Choose from the most used tags”.

When I click that what appears is “-1”.

I can add posts without problems and the site is generally running fine.