Sh ver output shows that the 2600 series router "uptime is 1 week, 16 hours, 59 minutes System returned to ROM by power-on...". Is there any command to show details of why/how the router lost power?
Short answer is no.
The router doesn't know how or why it lost power, it just knows it did and 1 week-16 hours 59 minutes ago the power was restored and it booted back up again.
Is the router powered by a UPS? Is that UPS connected to a Network Managment Centre or simular? this again would only really detect that you'd lost the grid feed and that report that the UPS was on battery and had some time remaining. it wouldn't give the how and why. but it could give you some time to get somebody to flick some breakers somewhere to restore power....