Does it not power up at all, or does it "just" fail to boot? In the latter case you may be able to fix it by reflashing, otherwise try testing the battery in another compatible device (for example E61i or E90).

It may also be that it originally failed to boot even to the battery charging runlevel, but that the battery has now been drained completely so it won't power up. You can try charging the battery fully in another device and then trying to flash the N810 with it. IIRC the bootloader will refuse to flash the device if the battery charge is less than 50% or so.

Sorry, I have no idea how to get nokia.gr to behave reasonably :-(