Some possible workarounds:

Restart bliss. Log into the VB using SSH and issue a restart command: service bliss restart and press enter.

An alternative is to restart the VB itself.

Finally, you could upgrade bliss, which performs the act of starting/restart bliss anyway. Press the 'install' button on the bliss packages page.

In terms of fixing the problem so it doesn't occur, I have seen this before and is caused where the stylesheet, for some reason, isn't downloaded.

Get in touch via email if you need to provide more in-depth debugging information: http://www.blisshq.com/contact.html