The server which renders the tiles has had an upgrade and is currently re-importing the planet, so it is unable to do tile rendering at the moment.
Of course this is a bit annoying: the upgrade should improve the speed with which tiles get rendered so we’ll benefit soon.
In the meantime you can try and get Osmarender to render your data or look at MapQuest (and probably some other sites) which use different rendering servers. Or you can render stuff locally using Mapertitive.
In the old days of a 2009 OSM was like this all the time, so I taught myself how to do local rendering so as to avoid having to wait a week to see my mistakes!