When first using Restangular on a working website I got the following JavaScript error:
Failed to instantiate module restangular due to: \'_\' is undefine
It's a simple oversight. The '_' (underscore) is a JavaScript utility library that Restangular uses and depends on. Include Lodash or Undrescore library before the AngularJS library (in your HTML):
or
PS: Double check the library script and path exist as referenced in your SCRIPT tag.
Additional readings: