Same thing happended to me last Novemeber, but I was using osCommerece then. That is also the same time that my hosting provider switched to PHP 5.2 and Mysql 5. Has your been upgraded recently?
Did you get a copy of the order from 2checkout? I was able to find the order by using Recover cart sales and changing the status it looks for to processing. I was then able to fill it. I could never get the 2checkout to work though. I just Switched my cart over to Zen and I have 2checkout running fine (as far as I know). Using the same hosting provider so I'm not sure why yours would stop working.
I know 2checkout was recently working on there checkout pages so you might want to check and see if they changed something. Guess I better do another test run on mine.
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