admin ajax php shows 0

I received a mail form wordpress team recently.It said my theme(Astra) is experiencing an error that causes fatal error on my site. I checked my admin ajax php,the website only shows 0 on the top left of the website. I got poor code technics and don’t know how to slove this problem, can somebody who has experienced the same situation tell me what should I to this kind of situation? Thanks!

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