Field exception message

I am getting the following message:
FieldException: Cannot change an existing field's type. in field_update_field() (line 234 of C:\inetpub\Drupal Site\modules\field\field.crud.inc). I can only access the admin pages but can't get to the admin/module pages and my site is showing up with a error 500 message.

How do I proceed to correct this issue?

Thanks,

Iris