word 2013 autocorrect stopped working mid-document

Just an educated guess:

Both AutoCorrect and the spelling checker are sensitive to the language applied to the text. In Word, the language is a property of (potentially) each character, like bold or underline. To complicate matters, there's a feature (some of us consider it a longstanding bug) that tries to guess which language you're writing in, and changes it on the fly. Text can also be tagged to prevent spelling and grammar checking, but that shouldn't have any effect on AutoCorrect.

To see if this is the problem, click in a word that's misspelled but not being caught by the spelling checker. On the Review tab, click Language > Set Proofing Language. If the language that's highlighted in the list isn't what you started with -- and especially if the check box for "Detect language automatically" is checked -- you've been bitten by the language bug. In that case, close the dialog, select the whole document (Ctrl+A), reopen the dialog, select the correct language, and clear both check boxes.

Another item to check is in File > Options > Proofing > AutoCorrect Options. On the AutoCorrect tab of the dialog, make sure the box for "Replace text as you type" is checked, That wouldn't have any effect on the spelling checker, but might explain the lack of AutoCorrect.

While you're on the Proofing page of the Options dialog, click the Recheck Document button to see if fixing the language setting has restored the spelling checker's function.