MacOS upgrade woes

I recently made the foolish error of updating my MacOS without proper preparation and now I’m in a pretty dire position.

It turns out I didn’t have enough disk space to upgrade to Monterey and so the install failed. I got caught in the failed install loop for a while before - after lots of internet research - I found a way to attempt installing Sierra (my Mac’s original OS). This didn’t work, and how my internal HD is completely full.

I’ve managed to install Sierra on an external USB key and can get into my computer. However, the internal HD refuses to mount. The volumes Preboot, Recovery, Update and VM mount, but the HD refuses.

All I want to do is get into the internal HD, retrieve my data, and then I’ll he happy to reformat and start again.

can anyone help?