iPad won't power on with a force restart or plugging in and charging for nearly 8 hours

Has your iPad been allowed to remain without charge for an extended period? If so, the battery may well be dead.

An iPad, similar to any other device that incorporates a rechargeable Li-ion battery, must have its battery properly maintained - otherwise the battery will irreversibly damaged by deep-discharge.

To quote directly from Apple...

Store it half-charged when you store it long term.

If you want to store your device long term, two key factors will affect the overall health of your battery: the environmental temperature and the percentage of charge on the battery when it’s powered down for storage. Therefore, we recommend the following:

• Do not fully charge or fully discharge your device’s battery — charge it to around 50%. If you store a device when its battery is fully discharged, the battery could fall into a deep discharge state, which renders it incapable of holding a charge. Conversely, if you store it fully charged for an extended period of time, the battery may lose some capacity, leading to shorter battery life.
• Power down the device to avoid additional battery use.
• Place your device in a cool, moisture-free environment that’s less than 32° C (90° F).
• If you plan to store your device for longer than six months, charge it to 50% every six months.

Depending on how long you store your device, it may be in a low-battery state when you remove it from long-term storage. After it’s removed from storage, it may require 20 minutes of charging with the original adapter before you can use it.

Batteries - Maximising Performance - Apple (UK)

You only hope, short of submitting your iPad for a “battery service”, is to leave the iPad connected to its Power Adapter for a couple of days - undisturbed. After a couple of days - and while still connected to the Power Adapter - attempt to restart the iPad.

If you are lucky, the iPad will restart. As for the condition of the battery, or if it will continue to operate at all without support of the Power Adapter, you will only discover through attempted use.

Charge and monitor the iPad battery - Apple Support

At worst, you will have tried and failed to recover the battery to some kind of usable state. If the battery has failed, a visit to the Genius Bar at your local Apple Store will provide opportunity for your iPad to be tested by the technicians - and for suitable advice to be offered as to your available options.