(Redirected) BIOS problems on 2720

In the course of running Windows 10 upgrades via the Dell drivers page, it recommended I flash an upgrade to the BIOS of my 2720 AIO, from A07 to A11. Ever since doing that I now have three problems:

1. In BIOS (or UEFI?) the screen now flashes on and off so that I have to struggle to see and change anything.

2. One of my three Windows 10 partitions (2 on an SSD and one on a hard drive) also flashes on and off, though not so quickly. I've reinstalled Windows 10 three times but to no avail.

3. Whenever I switch on, I get an error message saying the boot device has not been detected and inviting me to Press F1. As soon as I do this -- every time -- the machine then boots normally.

I still have a copy of the original BIOS and tried to revert to that but the machine won't let me, saying that a later BIOS is installed.

Coincidentally, and I'm not sure if this is connected, the 'sleep' option in all three Win 10 partitions inexplicably disappeared after the flash upgrade and is no longer available anywhere.

I'd truly appreciate any guidance. I'm not technical but moderately competent.