iPhone 8 will not charge.

If you already tried (to no avail) all the steps provided in this support article:--> If your iPhone or iPod touch won't charge, then yes, it could be a hardware (battery? other internal component?) failure but we, just users like you, cannot diagnose that issue remotely, sorry.

I suggest that you make an appointment (*) to avoid queues, and have your device+charger+cable inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> https://locate.apple.com/choose-country-region . After checking it out, they'll give you all the available options.

If it's a matter of battery, you can get an estimate here (select your country):--> Choose Your Country or Region - Apple Support

Regards

Giulio

(*) You can do it:

Online:--> Genius Bar Reservation and Apple Support Options - Apple;

Contacting a local Apple Store or AASP among the list provided above;

Using the "Apple Store" app, available and downloadable from AppStore.

My pleasure