Can't restart elastic search service on debian

On my debian VPS, I tried to restart my elasticsearch but it failed.
I have no trace on /var/log/elasticsearch.log.
I tried also to update my elasticsearch cluster but I received the following (sorry this is in french):
Erreur de GPG : stable Release : Les signatures suivantes n'ont pas pu être vérifiées car la clé publique n'est pas disponible : NO_PUBKEY D27D666CD88E42B4
This service often fall down and I don't understand why.
Is anyone can help me.
I'm on a Debian GNU/Linux 8.5 (jessie)

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