Power it down, then connect it to an Ethernet source using an Ethernet to USB-C adaptor, then power it up while holding Shift + Option + Command + R until you see a progress bar or a rotating globe. It should then boot into a version of Internet Recovery Mode. From there you might be able to tell whether or not it's Activation Locked to an Apple ID. If that procedure doesn't work you may need to do a full device restore by putting the device into DFU mode and connecting it to another computer running Apple Configurator. Good luck!