I'm using the standard version of MT, no MySQL or anything.
The entries are still there, as far as I can tell. Thank goodness...
My MT Technician (

) has been looking into this as well and he found that an export of entries quits at march 31st, even though there are entries for april.
It seems that the april 1st entry (since deleted) has broken the archive.
Just to see if it would work, I added a new entry for april 1st (midnight exactly) and rebuilt. No luck. Then changed the time for that entry and rebuilt yet again. No change. I then deleted the entry and rebuilt. No result.
With over 2050 entries, rebuilding again and again takes forever.
We're now contemplating starting from scratch with a new blog, importing the entries until march 31st and manually adding the april entries... but that is no guarantee either, since at this time it's unclear what is causing the problem in the first place.
I wonder... could downgrading help?