The router rebooted by itself or reloaded. How to find the route cause why it got rebooted.
XXXXXXX# sh version Cisco IOS Software, 1841 Software (C1841-IPBASE-M), Version 12.4(1c), RELEASE SOFTWARE (fc1) Technical Support: http://www.cisco.com/techsupport Copyright (c) 1986-2005 by Cisco Systems, Inc. Compiled Tue 25-Oct-05 17:10 by evmiller
ROM: System Bootstrap, Version 12.4(13r)T, RELEASE SOFTWARE (fc1)
XXXXXXX uptime is 2 days, 22 hours, 5 minutes System returned to ROM by power-on System restarted at 10:52:58 EST Fri Mar 7 2014 System image file is "flash:c1841-ipbase-mz.124-1c.bin"
Cisco 1841 (revision 7.0) with 97280K/33792K bytes of memory. Processor board ID FTX1206W03B 2 FastEthernet interfaces 1 Serial interface WIC T1-DSU DRAM configuration is 64 bits wide with parity disabled. 191K bytes of NVRAM. 31808K bytes of ATA CompactFlash (Read/Write)
There is no crash file on the logs, only thing is on the show version command the output shows System returned to ROM by power-on, that said you can infer that the router was either power-cycled, or that the power source went down for a few seconds. Verify your power source and troubleshoot the outlet circuit (power to router). See the link below: