Without warning, MyTopo.com tiles are not available as of Monday, Nov 21st.
This will not affect your previous downloads, but new preview or downloads will refer you to Trimble's apps.
I have talked with MyTopo and have not resolved the situation. Since they have been bought up by Trimble, they would like to put other outdoor apps out of business and drive traffic to Trimble apps. They will not provide their maps even if you or I are willing to pay.
UPDATE Nov 22nd
We are now offering USA Topo Maps, hosted by ArcGIS, as an alternative to MyTopo. Please use this source and you will probably like it.
Learn more about it here:
If you were using MyTopo maps in Canada, there is already a good alternative: NRCan topo maps of Canada.