I had numerous problems with win10 so I tried to uninstall to an earlier version(win 8). I am caught in the start up loop. No system disk to boot from. No valid system restore point. Start up repair is ot working. HDD diag is ok. Toshiba satellite laptop.
When it says it is looking for ntldr, it is looking at the MBR of a drive and expecting to find an XP type bootloader. Since Bootmgr has been used since Vista, possibly the bios is not seeing the correct drive.
If you cannot access the Bios or some type of Boot device menu to adjust the system to boot to a Windows Boot Manager, I suppose you are stuck... I have been unable to tell if such a situation is caused by the OEM boot setting utility being overwritten or something else. But try hitting the keys and do it very, very early in the boot scenario and keep tapping.
It is basically impossible for us to tell the exact situation, but if you could boot to any install media, it might give you some options. I don't think you are the only one recently to have this exact same situation.
If you were running Windows 7 and you have a sticker on your laptop that has your Windows Serial Number on it, then you may be able to download an ISO file for Windows 7, (you will have to know what version you had, Home, Pro, etc., 32 or 64 bit) then you can make a bootable install disk that will reinstall Windows 7 from scratch.
Select the ISO file as the source and the DVD drive as the target.
You should end up with a DVD that you can insert into your computer that will reinstall Windows 7.
I just noticed that you said that you updated from Windows 8, but tried to go back to Windows 7. If you were using the recovery application built into Windows 10 it will only go back to the OS it was updated from.
And the serial number on you Windows sticker on you computer will be for the operating system that was installed when the computer was new.
If it was 8 that's what you have to reinstall.
Here's some information on how to enter the bios setup on a Toshiba Laptop...