Do a search for sleep mode, there's some flags you can set to prevent your network from going to sleep or you can set a cron job to ping the gateway every minute (this is what I do).I have an issue with both my B+ and Model 2 where I occasionally find them locked up. The device doesn't respond to anything, HTTP, SSH and the Ethernet port is lit. It could be a kernel panic but I don't have anything attached to view the output.
EDIT: Found the link: http://raspberrypi.stackexchange.com/questions/1384/how-do-i-disable-suspend-mode/4518#4518
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