I bought a secondhand L46720G and it ran beautifully, but had a 500 GB drive and I wanted a 1TB. I used Acronis to clone the drive (including the UEFI) and then installed that to boot, but it wouldn't recognise the disk. After several tries, I removed the battery to reset the BIOS and since then nothing on the screen at all. Thinking I'd done something that stuffed the mobo, I tried a second one with the same config. The original booted fine with the monitor, but once I put the TB drive in, it also refused to boot form that or a CD/DVD for Acronis or Win 10 repair disk. That one will now show the BIOS screen after the battery is removed to force a reset, but then it saves the BIOS and reboots when I get nothing - power is on and the disk is spinning (or the DVD is seeking, if there's a disk in the drive), but nothing shows on the screen.
It seems that I've done something to stuff up the UEFI, but I don't know what - or if it's recoverable.