I've had a similar problem in finding my "Sent Mail" folder appearing empty.
It occurred to me recently that the operating system (for some reason) may have changed the "Read" permissions on the files or folders within Mail.
I ran "Disk Utility" and had it "Repair Permissions" on my Mac's hard drive (i.e., "Macintosh HD") and this seemed to fix the problem.
You might want to try this solution.
Since upgrading my wife's iMac to Mavericks, all of her e-mails are blank; they will show as many lines as one wishes in "preview" mode; but if one goes to classic mode, or if one clicks on the e-mail to read it fully or to reply, they are blank. I've reindexed; used Hazem Malek's technique; cleaned up permissions; all to no avail. Exported mboxes and reimported; the problem is very persistent and very irritating. I'm a long-time (1990) Apple user, and this is the worst experience with Apple yet, in nearly 25 years. If someone finds the silver bullet, please pass it along.
If Mail detects problems with information in your mailboxes, it may need to repair the mailboxes by reindexing your messages.
At other times, you may want to reindex messages yourself—for example, when searching mailboxes using Subject, To, or From doesn’t return the correct results.
Weird. Same thing happened to me but before going crazy, (all these things drive me nutso) I restarted the apple and that seems to have fixed so far. Sometimes too, it helps to delete the email from mail then add it again. I also did the simple solution above which did not work for me. I was ready to go off and do bunches of fixes but try restarting the entire mac first before doing all the fixes. Apples often have the habit of fixing quirks on restart. Anyway thanks for all the good information.