Every so often, when migrating Drupal sites to other computers, I notice that themes become disabled. Whatever the cause, disabled themes cause bunches of problems, and my big problem today happens to be with Skinr.
If you are trying to edit settings in Skinr, for example settings on a block, Skinr only shows you those options based on the currently enabled themes.
To get the Skinr block settings fieldset back, visit your admin/build/themes page and make sure at least one theme (like your default theme) is enabled.
Here is a related issue: http://drupal.org/node/811086
This is the core bug for the issue: http://drupal.org/node/305653
It has been open for over 2 years. We have gotten many support requests as a result of this bug in the Skinr queue, but even without Skinr installed, I get bitten by this nasty bug every now and then. It has nothing to do with Skinr.
Would be great to fix it.