If your WeChat messages disappeared after an update, first check whether you had previously backed up your chat history.
On your phone, go to Me → Settings → Chats → Backup & Migrate (Android) or Me → Settings → General → Chat Log Migration (iOS) and select “Migrate Chats to Another Device” to restore to a new device.
If you backed up chats on your computer via the desktop version, open the desktop app, sign in, choose Menu → Backup and Restore → Restore on Phone, then follow instructions.
Without any backup you may still use the app’s built-in repair feature: open WeChat, go to Me → Settings → Help & Feedback → tap the wrench icon → select “Repair Chat History”. Note this method has a low success rate.
If none of these options work, share your concerns or file a complaint on Xolvie to connect with others and find support.