Last night, I went into my kitchen and discovered the kitchen/porch light switch no longer functioned...?

While it's not likely that the kitchen/porch lights are on a GFI, it's entirely possible that the bathroom and part of the kitchen (the lights)are on one circuit, especially if they are adjoining rooms. The bathroom outlet could be GFI protected and that would affect the kitchen light if it's tripped. If there is no tripped GFI go back to the service entrance. The breakers should be labeled but if not you can find the one that serves the culprit circuit by the process of elimination. Turn them off one by one and note which circuit goes dead. The one you turn off that doesn't affect any other part of the house is the one you need to trouble shoot. Circuit breakers and GFI's do go bad occasionally.