Git- made changes on a local branch, checked out master, and master is now broken
Possible causes:

(Git) You forgot to git add files in the conceptCalendar branch, and they are still lying around when you checkout master.
(Python) You have stale .pyc files in your project. Remove them.
(Django) You forgot to makemigrations in the conceptCalendar branch
(Django) You ran migrate on the conceptCalendar branch, your database schema has changed, but now the code on master reflects the old schema. Rebuild your database, or migrate backwards.

I'm betting my money on this last point. From the error you posted, i'm thinking that maybe a Form is extending ModelForm for a Model that changed in the other branch. Check that all fields exist in the underlying model, and in the database.
I am working on a Python/ Django project, using Git to manage my version control.
I recently made some changes on a branch called conceptCalendar3, and the changes I made broke my site.
I committed the changes to that branch, and then checked out master, which, I had branched from in order to create the conceptCalendar3 branch. However, when I now to try to view my site from the localhost, on master branch (on which I have not made any changes since it was last working), I now get a message in the browser telling me that:

This site can't be reached
localhost refused to connect

The Python console is displaying a lot of output with error messages that I've not seen before:

File "/Users/.../Documents/Dev/moonhub/moon/moon/urls.py", line 27, in 
      url(r'^costing/', include('costing.urls', namespace="costing")),
    File "/Users/.../.virtualenvs/moon/lib/python2.7/site-packages/django/conf/urls/init.py", line 52, in include
      urlconf_module = import_module(urlconf_module)
    File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/importlib/init.py", line 37, in import_module
      import(name)
    File "/Users/.../Documents/Dev/moonhub/moon/costing/urls.py", line 2, in 
      from . import views
    File "/Users/.../Documents/Dev/moonhub/moon/costing/views.py", line 2900, in 
      from projects.views import get_project_folder
    File "/Users/elgan/Documents/Dev/moonhub/moon/projects/views.py", line 38, in 
      from .forms import *
    File "/Users/.../Documents/Dev/moonhub/moon/projects/forms.py", line 1207, in 
      class PostDepMeetingForm(ValidatedForm):
    File "/Users/.../.virtualenvs/moon/lib/python2.7/site-packages/django/forms/models.py", line 257, in new
      raise FieldError(message)
  django.core.exceptions.FieldError: Unknown field(s) (meeting_date_time) specified for Survey

The 'field' that it seems to be complaining about, meeting_date_time is one that I added on the conceptCalendar3 branch- but it doesn't exist in the code on the master branch...
I have tried running git pull origin master to ensure that I have the latest version of code from the live version of the project, but this tells me that everything is up-to-date.
So why can't I view a version of my site locally, and why am I getting these errors in the console?
Edit
I tried checking out an old commit, and at one point was in a detached head state- could it be that I am still in this detached head state, and so some of my code is point to master, but some of it is pointing to conceptCalendar? If that's the case, how would I check, and how would I resolve it?