Hi,
Is there any way to detect if a router was Down due a power outage using NPM/SAM ?
yes.
You don't say what model of router.
So, assuming a cisco: two ways.
a) the output of show version
b) the SNMP OID 1.3.6.1.4.1.9.2.1.2 (whyReload)
a SNMP coldStart trap (though since routing is normally not up yet the chances of receiving this are very small)