This whole thing is so annoying. I have zigbee repeaters right next to the zb device and still no line.
I have this problem as well, but seem to have found a workaround.
UPDATE: Nope, turns out this isn’t a reliable way to restore the link. I lost the link again this afternoon, and doing this has not helped. 
I believe if a device is “asleep” to conserve battery when the topology is read by the system, the link disappears.
This seems to work: wake up the device (on my Aqara temperature sensor where the link is often missing, pressing the button once to force it to wake up and send an update is sufficient). Confirm on the device page that an update was received to validate that the device is actually on the network.
Once that is done, press the “Refresh Topology” button on the ZHA network visualization, give it a second or two, and then refresh the page. The link should now be visible.
Anyone found what causes this? Just switched from Deconz to ZHA (new Zigbee controller). In Deconz mesh worked fine and updated frequently but in ZHA i get this:
Edit:
Solution was to remove the Deconz addon and service. Weird though that having Deconz addon would cause links not being displayed in ZHA?