Yup, happens with me and at least a few other people over the past couple days. Probably a Steam issue. To fix, try deleting clientregistry.blob. If that doesn't work, then Steam just may be working on the issue. Try looking under your Favorites tab or History tab for a list of servers.
Someone asked me why I have an advert on my server telling people add a server to favorites when there are only a few active ones? Well this is why.
