Possibly been hacked?

Hi,
I have been mostly offline due to the Covid for the last year and am now trying to get going again with my website. However, I realise that I have possibly been hacked? When anyone tries to load my website, after a short few seconds, an advert appears and redirects you from my page… then you need to reload my page again to try again to access. I can still access my backend.
I have updated all plugins, I have changed my password. I’m not too sure what else that I can do.
Any help would be great
Thank you
PS I’m not terribly techy so basic language would be great …

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