The most common culprit is a full storage quota. Go to Settings → [your name] → iCloud and check how much space is left. If you're at the limit, new files simply stop syncing without any clear error. Free up space or upgrade your plan. Also make sure iCloud Drive is toggled on for the specific app that's not syncing — each app has its own switch.
Check that iCloud Photos is enabled in Settings → Photos. If it is, look at the status bar at the bottom of the Photos app — it usually shows how many items are left to upload. A slow or unstable Wi-Fi connection is often the reason uploads stall. The service requires a stable connection; mobile data works too, but only if you've allowed it under Settings → Photos → Mobile Data.
First check Apple's system status page to rule out a server outage. If everything is green on their end, remove the iCloud mail account from your device and add it back. Also verify that SMTP settings haven't been changed — the outgoing server should be smtp.mail.me.com on port 587.
A failed payment in Apple's ecosystem usually means the billing details on your Apple ID need updating. Go to Settings → [your name] → Payment & Shipping and re-enter your card details or switch to a different payment method. If the card is valid but still gets declined, contact your bank — Apple's charge descriptor sometimes triggers fraud filters.
This is almost always a browser cache issue. Clear the cache and cookies for icloud.com specifically, or try an incognito window. Safari and Chrome tend to work best; Firefox occasionally has rendering issues with the web interface. If the problem persists, disabling browser extensions one by one usually identifies the conflict.