Since Microsoft have released the Windows 10 - 1803 update we have had a few customers complain that their computer no longer boots into Windows.
This is caused by an older machine needing a BIOS update.
We would suggest that if you are running Windows 10 to update your BIOS as soon as possible before the 1803 update is automatically installed.
To receive your BIOS update you would need to visit your computer manufacturers website to download this.
As in all situations like this we recommend you backup your data before performing this procedure.
Here are a couple of links to the main Manufacturers we supply, these utilities help you to update your BIOS.