[nmglug] Lost grub, can't boot; rescue or replace via chroot?

[Tom Ashcraft]

I'm trying to save my newish installation of Debian 10 Xfce on an old
64-bit AMD Toshiba Satellite L505 laptop.  The recently damaged boot
routine drops to 'grub rescue >'.  So far, the suggested remedies I can
find for working from 'grub rescue >' have not worked out; however, I'm
given to understand that alternative strategies might be more
appropriate.  

To wit, I have both a live USB stick and a persistent installation of
Debain 10 Xfce on a USB stick that (except for a great deal of data and
installed software) is virtually identical to the damaged installation.
I made this for occassional use on another laptop of similar vintage. 
It was created and runs fine on the computer with the boot problem. 
>From the desktop of the persistent USB, I can easily see and manipulate
my HDD files.  I can also see and contrast side-by-side the nearly
identical files of the working system with the damaged system. (Not
that it yet helps much.)  

My take from what I've read is that, theoretically, it ought to be
pretty simple to get in there and 'chroot' into the broken installation
and update or reinstall grub.  

The problem is that I don't have the experience to understand exactly
what I'm doing or properly interpret the generic instructions I've
found, so I keep falling down rabbit holes. 

>From the sort of error messages I've been getting, it seems that in
using 'chroot' I'm having trouble understanding how to exactly acertain
where I am.  The prompt leads me to suspect I'm still actually  working
in the system on the USB stick.  Beyond that, I apparently don't know
how I should properly mount the partition (or partitions) I'm trying to
be root in.  

How lvm and mbr and efi partitions do or do not interact with all this
confuses me too.  The installation is lvm on a single partition, I
believe.

Any suggestions?

Tom