I would suggest unplugging the battery from the back to get it out of boot-up mode. Then plug it back in with Calibre closed, open it up like USB and pull the books you loaded off it. Something copied over that it didn't like.
You already updated to 2.1, right? I tried the 1.6 Mobi firmware first and it did some wonky things (4 resets in a hour!), so I switched to 2.1 ePub and haven't had any problems since.
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