Hello Stephen,
Is she sure her MacBook can be upgraded to Mojave?
MacBook (Early 2015 or newer)
How much space did she have prior to installing Mojave?
How far did the installation get to, did she receive an error message?
The MacBook is connected to Power?
She could try: Boot into Recovery Mode:

Turn off the MacBook
Turn it on and hold Command-R until you hear the startup chime or see the Apple 
logo.
Click Disk Utility when the macOS Utilities window shows up.
Click the disk that's running Mojave.
Click Repair Disk.
Wait for the repair to run. The MacBook will likely restart when it's finished.

Regards,

Ronni

 Ronni Brown’s iPad Pro 12.9-inch 256GB 


> On 19 Jan 2019, at 5:49 pm, Stephen Chape <[email protected]> wrote:
> 
> Hi folks.
> 
> A friend just called to tell me the above.
> She had left her MB upgrading to Mojave and when she returned to the MacBook 
> the screen was blank.
> Assuming it had shut down after upgrading, she tried to power up, but NO 
> response.
> 
> Tried stuff like holding down Shift key at start up (safe mode) - nothing.
> And holding down Shift, Control and Option at start up - nothing.
> 
> She tells me the green light is on so it seems to have power.
> 
> Any ideas please ?
> 
> Regards,
> Stephen Chape
> 
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