Do you see javascript errors? nothing should have changed recently that could cause such problems. Maybe you get more traffic on your piwik? have you setup: http://piwik.org/docs/setup-auto-archiving/

Attachment: The screen when logged in into piwik
PIWIK screen.jpg

I do not see javascript errors in firefox. I will try this with Chrome.
There is little traffic on my site, so that's not a issue.
Auto-archiving is off.

With Chrome debugging I see the following log:

GET http://www.xxxxx.net/piwik/index.php?module=Proxy&action=getCss&cb=9c5da3a1006a2c4ff7ff2c69f58bd1d0 500 (Internal Server Error) index.php?module=CoreHome&action=index&idSite=1&period=range&date=last7:45
Plugin file loaded plugin.js?cb=9c5da3a1006a2c4ff7ff2c69f58bd1d0:18

Ok error 500 is interesting, it means your server could not finish the request. please check your server error logs for more information.

Also this workaround may work for you (but piwik will be slower than actually fixing error you'll find in your server error log)

http://piwik.org/faq/troubleshooting/faq_135/

Thanks for the support. The apache logs showed that the GEOIP-database could not be found. I fixed that.
Also changed the execution time to 45 seconds.
Now it is working again !