iPad Pro 10.5 inch won't turn on after blacking out

Given its age, the iPad Pro 10.5" has very little value for Apple's trade-in program - and unless the iPad can be successfully restarted so as to demonstrate that the iPad is actually functional, its already negligible value will be reduced to nil. You can obtain an estimated trade-in value here:

Apple Trade In - Apple

Given your description and the age of your iPad Pro 10.5, it is perhaps likely that its battery has reached the end of its usable life.

Are you able to restart the iPad while it remains connected to its Power Adapter. If yes, while monitoring the indicated battery state, disconnect the Power Adapter; if the indicated battery state suddenly falls, or the iPad shuts-off, this test demonstrates the iPad battery having reached the end of its usable life:

iPad Repair & Service - Apple Support

Otherwise, these support pages should help with troubleshooting battery and charging issues:

Charge and monitor the iPad battery - Apple Support

If your iPad won't charge – Apple Support

Troubleshooting generally requires substitution of the Power Adapter and cable for another known-good item. Most models of iPad require a Power Adapter rated at 12W or greater to reliably charge. 5W Power Adapters intended for older models of iPhone are inadequate to charge an iPad; if the iPad charges at all, charging will be very slow.

Be aware that many computer USB ports cannot source sufficient power to charge an iPad. Unless explicitly designed for charging connected devices, computer USB ports are typically limited to 0.5-1A @5V (i.e., 2.5-5.0W).

If you continue to experience difficulties when using an appropriate Power Adapter of 12W or greater, you might be best advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad:

Genius Bar Reservation and Apple Support Options - Apple