Try to soak the guide area with some penetrating oil. Also try using something to gently pry the valve back up. Be carefull not to bend the valve stem. Also might try to get the spring off where you could get hold of the very top of the valve with some vise grips and try to turn the stem. Be very carefull not to loose the valve down in the cyl. Also don't scar up the surface of the stem.
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