Same issue for 2 days. iOS app, V1 Hub, Echo devices. The following steps from Smartthings customer service helped me. Probably they have fixed the issue for some.
- Uninstall and reinstall the SmartThings app
- Disconnect the power and ethernet cables from the Hub
- Wait at least 30 seconds
- Reconnect the power and ethernet cables
- When the Hub is back online, its LED will turn solid green