I don't know if this is the same, but I did have something similar. in the Admin, I could see the order, but couldn't see the lines.
The order actually got duplicated.
This was because when the procedure for Adding to Cart (I think its AddToCart...??) was running, I had another program running to calculate their price for the cart with discounts at the same time. We didn't have a NOLOCK on the request, so it created another order and left the other order half done.
So...I'm guessing that something is getting held up. Is your site on a SMTP relay? I know we had to add the server to our email server. now I just use gmail for everything. Free and works (mostly)
if you find out what happened, let me know. I try to keep a running list of things of "enhancements"