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Unfortunately there are no hard reset feature for S40 devices so if it won't boot you will have to visit a Nokia care point. If you already have disconnected it from computer, please try to remove the battery first and giv it a cold boot after reinstalling the battery.
If it however seems to react to connecting through usb, check if you can trick the pc to discover it and hopefully you will be able to run the update again. Unfortunately this is not likely to work and depends on the state the phone was in during the failed update. If it's "dead" - it is dead and you will have to look here for your nearest care point: http://europe.nokia.com/support/repair-and-recycle/repair/care-points