Most people will say that “this must be caused by one of your plugin”, because as far as everyone understood, Wordpress’ core module is much stable upon its release (unless you using their beta version, and also if there’s any)
For starter, please look back at what you’ve just done to your site. By look back, I mean thinking of any changes that you have done, like (of course) installing plugin, settings, performing pentesting like detectify or something similar. You name it.
Or perhaps it’s neither you nor Wordpress fault at all, but rather your site is under “attack” by someone. In this case, then you may need additional protection to your site, like Wordfence or using Cloudflare (I’d recommend Wordfence if you weren’t the one who set up DNS’es between your site and domain name)