After one annoying google search, It seems that Android was using YAFFS2 up until 2.3, and ext4 from then on.
First, I would Plug the phone into the computer without auto mounting, and copy the entire internal storage chip with dd (ie. `dd if=/dev/androidsdx1 of=/android.phonename.yyyy.mm.dd.fstype.img'), and perform ext/yaffs file recovery with a copy of the image (dd even copies the unused space). And if all goes well, you can copy the file(s) manually, or perform the recovery on the phone itself (I would do the former with `cp -prv').
If that fails, I would see if the appstore has the same texting app, and reinstall it.
And if that fails and you do mind about lost data (reflashing the phone (of course, you made an image, so you could theoretically restore the data)), I would copy the file(s) from a friend's rooted phone.
And if all else fails, I would just reflash the phone completely.