You cannot access your grid in Conga Grid or see a perpetual loading spinner. If you see the spinner and you open your browser console, you might see an "INVALID_FIELD" error like the following example:
You cannot access your Conga Grid because permission settings are not properly configured. You must be included in the Conga Grid Users permission set.
To check that this is the issue you are experiencing, select the F12 key to view the error and to add yourself to the Conga Grid User Permission set. You can also ensure all necessary parties have the correct permission set enabled.
After Conga Grid is installed in an Org, you need to give your Users access to the app. To give your Users access, you must assign Permission Sets to the Users at the Profile level and grant them a Conga Grid license. For more information on how to enable access, see Manage Licenses and Assign Permission Sets.