For those of you experiencing problems connecting to the steam client this morning - this appears to be caused by a Steam client update. The first thing you'll want to try is deleting your ClientRegistry.blob file, and then re-start steam. You'll find it in your steam bottle:
/[Steambottle]/drive_c/Program Files/Steam/ClientRegistry.blob
Upon re-staring steam, you should be able to connect just fine, after your client updates itself (which might take a few minutes).