I've done that about twice today, and a couple of times yesterday - it's no biggy, I just move the map, do something else while it loads, and go and look at it.
Yep, that's how I know it... Grey segments... Sometimes they disappear, sometimes they remain for ages. A general software rule is that reloading something again and again doesn't help - it even increases total loading times - but with this issue it's different. Suddently the grey segments disappear - minimum the segment of the specific map segment needed. However, you need a lot of luck...
btw: It's a general issue also happening on wikimapia.org, not a CHM/TM-specific issue. And it's only Mapnik.