Does anyone know how to enable the autocorrect function that corrects spelling and punctuation?

There is an autotext feature that will change certain words to new words. For example if you type Il it will change it to I'll, etc. That is enabled by default and can't be turned off except for sms messages, if desired.

There is no spell check or punctuation check (except for a period at the end of a sentence when you hit the space bar twice.)

I thought it was a default function but mine stopped working for some reason.

Wirelessly posted (8700g: BlackBerry8800/4.2.1 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/102)

There are many good spellcheckers for BlackBerry, and many many posts about such over in the Aftermarket Software section of this forum.
BBCorrector
eSpell
BBspell
Instaspell
And others

What exactly stopped working? You can look in Options >AutoText and see what's in the list.