AH! That sounds like the old known bug, that the display and touchpanel is inactive. I have seen it a few times on Lineage - though more seldom that I did on stock, before it was patched, If you open the device, you will see that e.g. Caps is reacting, if this is the bug.
When I had it I held Power 1-2 secs, pressed Tab Trice and Enter (To open the power menu, and goto "Restart").