I imagine this is: Press Ctrl+C to cancel filesystem checks in progress.
This is part of the normal Linux boot process i.e. check the file system integrity every X boots / every Y days. You can configure X and Y, including suppressing the check entirely but …
It isn’t normally a problem. In fact it’s a good thing. You just let it finish.
Where it is a problem is if it never finishes. Is that what is happening?
Do you have a µSD card in the Librem 5?
Linux checks each file system according to the individual file system’s settings. So if you have two file systems (for example, the main file system on the internal eMMC drive and the file system on the µSD card), you can end up with the boot stopping to check a file system quite often (and in that case I choose to synchronise the checks so that they occur together or not at all).
In principle you could try connecting a real keyboard via USB - or docking.