Unable to turn on MacBook Air

Hello. Please help!

In order to factory reset MacBook Air A1466, I have erased the startup disk via the Disk Utility app, then I have gone back to the MacOS Utilities menu and selected Restore MacOS (Yosemite 10.10). This prompted for Apple sign in, so I handed over to my girlfriend for her to sign in. She clicked on 'forgot my password' and the screen froze on the 'signing in...' notification for about 45 mins. At this point, I held down the power button until the MacBook shut down. This doesn't appear to have been the smartest move however as for some reason, it won't switch on at all!

The MacBook had about 90% charge. I have tried holding the power button down for 10-20 seconds, but nothing. It is being completely unresponsive, with no indication that it is attempting to switch on / start up.

Any advice any of you could give would be greatly appreciated!

Thanks

Roman