I have a short lists of some issues that need to be looked for first before going nuts and asking the same old questions again and again.
If you are having trouble with your N800, please do the following:

1)Remove your battery, and both memory cards. Wait five minutes.
2)Reinstall the battery and cover, leave the cards out.
3)Press and hold the power button until the device turns on.
4)If it doesn't power up, plug in the charger and hold the power button again.
5)once it's booted, hold the power button in again to shut it back off.
6)Replace the internal card. Use the stylus to move the little metal tab over the memory card to hold it in place (I think some people aren't even aware this feature exists, so their cards jiggle out of place and error out.)
7)Try booting it again. If it doesn't boot or it can't see the card, then the internal card might be bad.
8)Stick the external card back in and go.
If the external card gives you trouble, you can power down and swap card position to see if it's the slot or the card. Look into the hole for the external card to see if any debris or pocket lint lives in there. Blow it out and do not use anything metal in that hole!

Nice summary. Hopefully it'll save some people some trouble. Fortunately I haven't had any boot problems with my N800 since I upgraded from the second version of OS2007 (but back then it was fixed by booting with the internal card out, replacing it afterwards).