Smartlead is sensitive to brief server-availability blips, and a single failed connection can disconnect your inboxes. (This is specific to Smartlead's own connection status — if mailboxes or domains are showing disconnected or inactive in your Maildoso dashboard itself, see Why are my mailboxes or domains suddenly disconnected, inactive, or missing? instead.)
If your accounts show as disconnected in Smartlead, export a fresh CSV from your Maildoso dashboard (Email Accounts → select the affected mailboxes → Export Accounts → Download selected accounts as CSV → Smartlead format) and re-upload it in Smartlead. This re-syncs the latest mailbox configuration and resolves most disconnections. Reconnecting via an old/previously-downloaded CSV usually does not fix it.
If they won't reconnect, let support know which mailboxes are affected and we'll help. For other common Smartlead/Instantly connection errors, see Troubleshooting sequencer connection errors (Instantly, Smartlead, Snovio, Email Bison, PlusVibe).