OK, I chatted with the Apple support team and I must say they were brilliant. The problem wasn't on my iCloud as I could open the files from my iMac. After deleting, then reinstalling the app from the App Store made no difference, I was advised to force a restart (Volume +, Volume - and side button until the Apple logo appears) and that solved it. I still do not know where the problem came from in the first place but everything works now. I hope this can help someone else, as this place was my first port of call (it has already helped me a few times, just by reading the answers to a problem I faced)