Lost my DietPi installation

Hi. I bought an amplifier/DAC that has an UP Core installed with DietPi used as a Roon bridge.

It was working great for a few years. I just turn it on and it quickly appears in Roon. However I recently disconnected it from power for a couple of weeks and now it doesn’t appear in Roon.

I connected a monitor and found that it boots straight into BIOS as it can’t see the DietPi installation. I can still see the eMMC under South Bridge in the BIOS.

Is there anyway to restore the installation or do I need to reinstall DietPi and Roon Bridge? I’m very new to this so would prefer if there was an easy way to fix what was already installed.

Thanks very much in advance.