Nothing is deleted and nothing is locked. When you reach your plan's storage:
- new files and changes stop uploading,
- everything you already have stays readable on all your devices,
- you can still download all your data.
Before that happens, Settings → Plan and billing shows a warning when storage is almost full.
How to fix it
- Delete large attachments you no longer need, or move them to Google Drive.
- Upgrade to Pro (1 GB) or Business (4 GB per seat). Sync starts again right away.