Additional sign-in or account-related issues within Microsoft Teams for business
Thanks Juan Pedro.
I have checked that I have the latest OS update (macOS Catalina Version 10.15.4) and I have tried all of your suggestions - and it's still stuck on 'loading'. I have gone onto my Macbook Air and I am able to load the app on there, so it seems that it's my iMac that doesn't like it. I have googled about this and tried various things but still not loading.
If anyone knows how I can find a solution I'd be VERY grateful! :-)