With the  drive connected, press F12 a few times at powerup until the boot menu appears.  Choose the drive and press enter.

@ejn63​ Thanks 10 Elder

My Optiplex3020 is approx. 10 years old and won't start Windows hence my wish to boot from Recovery USB stick. Pressing F12 gives several choices but not USB and in Boot sequence (Press F2) the USB is the first device. I have set "Secure boot" to OFF

I must be missing something!

What OS is on the flash drive, and what mode is the system running in (UEFI or Legacy/CSM ON)?

Won't start Windows - Does that mean you still get the Dell splash screen or is it completely dead?

Try replacing the size 2032 CMOS battery and go from there.  Bad batteries affect BIOS and therefore PC operation.  I couldn't even reload Win10 with a bad battery.

thanks ejn 63 and bradthetechnut

OS on USB stick is Windows 10

The F2 screen shows "UEFI Boot Path Security set to "never" so I assume system is UEFI ?

On pressing the ON button I can get into F1 and F12 screens or if allow PC to try to start and I get Windows Boot manager

I will replace the CMOS battery and report back

Was the flash drive prepared using the Windows media creation tool?

I followed the windows or Dells site advice but don't remember details

Have replaced the CMOS battery got warning time config wrong. Restarted now Windows Boot manager says

............ 

File:     /windows/system32/winload

Status:    0xc000000e

Please advise what to do now

If the system doesn't see the boot option, the drive is not UEFI bootable.  Prepare a new boot flash drive with the latest version of WIndows 10 using the MS Media Creation tool.

https://www.microsoft.com/en-us/software-download/windows10

Thanks ejn63

now running the recovery  -   from another USB stick  -  tried startup repair  - didn't help

It looks scary!

A want to keep my data files

Suggestions please

Before you do anything else, remove the drive and mount it into a USB box or adapter, connect it to a working system and back up and verify the files you need.

Then proceed either with trying again, or doing a clean install.