Ruby on Rails missing Thor

Question

When trying to run rails g migration a, I get the following message

Thor is not available.
If you ran this command from a git checkout of Rails,
please make sure thor is installed, and run this command
as `ruby script/rails

Now few days this did work, and I did check my gemfile and I find thor like this

thor (0.16.0)

but it doesn't seem to work, what should I do?