Dell Venue 7140 - No Bootable Devices found error

ok - here goes.... Switching on the tablet I get the 'No Bootable Devices found' error. I've ran a full diagnostics test and no issues found. I've tried switching the boot sequence between Legacy and UEFI back and forth, each time trying a reboot. Still no success. The previous night all was fine, it was left switched on and this morning it had seemed to have powered down and and then started complaining that the power source was not of the expected voltage. From then the error with booting it up started. There is now no warning about the power source anymore - I checked and made sure all the connections were good. Nothing apart from the power cable nothing else is plugged into it. No new software updates have been applied that I'm aware of. I've done searches for this issue and its starting to look like I might need to reinstall windows 10. Any other ideas - anyone? Thanks in advance!