If the field was present before and has randomly disappeared, there's a chance that someone on the account has made the field inactive.
1. Go to your Farm that the field is missing
2. In the left hand menu open the Fields menu
3. You should see the missing field with the status as Inactive (If not, your field was deleted by someone. You can contact support@swatmaps.com to help restore that field)
4. Open Field
5. Select on the checkbox beside Active