Since Friday 13th June Flickr tags have not been working properly and all searches by tag using the API are returning no photos.
Flickr have acknowledged the issue and are working on it.
Fetching favorites, groups, galleries and photosets is unaffected.
I switched the widgets in the sidebar to use photosets because when using search by tag there are no photos. However there are dozens of examples on this site that use searching of photos by tag and none of these is showing any photos right now.
So for example rather than use
slickr-flickr tag="dogs", instead I used
slickr-flickr search="sets" set="72157623501712321" type="slideshow" size="small" which pulls photos from my photoset that has most of my dog photos
Of course this is just a temporary fix until Flickr resolve their issues with their tagging system.