Hello,
Please restore your Anki collection from a backup made before this occurred: How to Restore from Backup
If this happened while you were trying to sync on your mobile device, you may have been asked whether you wanted to upload to or download from AnkiWeb. If you chose to upload, you likely overwrote your entire collection in the cloud. Simply restore from a backup, and please be careful when choosing whether to upload or download to AnkiWeb in the future.
As a reminder, if you choose to upload, everything on your current device will overwrite the cloud. If you choose to download, everything in the cloud will overwrite what is on your current device. Always make sure you know which version is the most up-to-date.
If you are having any errors in the process, please tell me exactly when the error is happening, and paste it here. Knowing how the error got triggered is essential to understand what’s happening.