Sleep key stops working after some time

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http://unix.stackexchange.com – I've got a "Sleep" key on my keyboard that I use to suspend the system when I'm away from the computer for a longer time. This works fine, at least for a while after I've turned the computer on. However, after some time, the Sleep key just stops working, and I've got to reboot the computer to make it work again. Problem is, I don't know what to check. What kinds of things can change in a system to disrupt the operation of this key? And sometimes it takes longer than other times before it stops working; sometimes I can use the key for days on end; other times it won't even work once. I can st (HowTos)