I often find that after recreating a Drupal site locally to do some testing that I have left CSS and JS aggregation turned on. This can be turned off easily enough via the performance page, but this often breaks the flow of what I am doing. As an alternative I use Drush to reset the values via the command line.
drush variable-set preprocess_js 0 drush variable-set preprocess_css 0
This can be shortened by using vset, which is Drush shortcut for variable-set.
drush vset preprocess_js 0 drush vset preprocess_css 0
To see what the existing value of a variable is just the variable-get command. This can also be shortened to vget.
drush vget preprocess_js
drush cc css-js
If you have anonymous page caching turned on then you might also need to disable that. This can be done by setting the cache variable to 0.
drush variable-set cache 0