This is an issue I've been experiencing recently after waking the iMac from sleep. It seems odd as really the settings don't need anything doing to them, if I click on the little wifi symbol on the top comms bar it shows my router is actually connected but safari says not. So I click on another for two seconds, reselect my own and hey presto back up again in seconds.
It's a mystery but I think that the mac is sensitive to interruptions in the ISP connection, these may not necessarily happen whilst the mac is online but still require a little fiddling with after the event. I've only concluded this by turning my router off when then iMac was put to sleep and once back up again turning on the iMac.