If packing slips won't print from the guns using Fedex as carrier, open 'Customer Shipment Tracker' and choose one with status 'Open' then change 'Ship Via' from FedEx Next Day to DX Next Day and tick the Shipped check box, and see if it ships. If it does it isolates the issue to Fedex. Make sure to unship it.
At this stage it could be either the API between us and Fedex or Fedex having an internal issue.
We can try to use postman to check if the API is working, but likely not if its down.
In this case raise a ticket through the Fedex portal using the following link: https://fedexeurope.my.salesforce-sites.com/premiumCT/, choose English as the language and enter the Fedex account number as 345818898. Enter the information as requested and click submit. This will give you a case reference which you can email to our Fedex account manager Darren Gleave (dgleave@fedex.com), and Daniel Ogley (daniel.ogley@fedex.com) for a quicker response.