Had the same problem with MX450.
The device showed to be offline. There were no problems with Internet or something else, since the Spare device was on the same internet line with different IP address.
Spare device started to send VRRP announcements.
We have rebooted the device, the opened a TAC case.
We were told that the OOB logs have to be collected from the Local Status Page before rebooting the device, otherwise they can't find what was the problem.
What I am not sure, if the status page was accessible at the moment of restart - I believe it wasn't, but can't confirm as we had no access to the device in that period.