I have an old iPhone 8 and won't turn on at all

We can't diagnose your issue remotely, but you can check:

1. If the screen is black, but the device is on, try a forced restart:--> Force restart iPhone – Apple Support
2. If the device won't charge:--> If your iPhone, iPad, or iPod touch won't charge - Apple Support
3. Check also your battery health: Settings>Battery>Battery Health and tap there, reporting here the value you can read (it must be >80%).

If you get no help, have your device inspected at an Apple Store or AASP:--> Apple – Location Selector

Regards

Giulio

Start here: https://support.apple.com/en-us/HT201412