A couple of times since installation of kit kat, the nexus did not power on. I had to hold the power button down for appx 8 seconds and then it came on. Is this a kit Kat problem? I never had this problem before.
I haven't heard this from anyone I know with a Nexus 7. Are you sure no other applications were installed or updated? I would wait a bit where hopefully someone will chime in but maybe a factory reset is in order