If your Gmail account has been disconnected from Reply due to an error (e.g. Invalid credentials), you can simply reconnect it:
Open the Email Accounts tab of the Settings page
Click on the arrow to the right of the email account string
Open the drop-down menu
Select Reconnect
Go to your sequence -> Settings -> General settings and re-assign the email account for sending
The sequence will be resumed within 10-15 minutes according to your schedule.
Please note that you cannot connect the same email address twice, and we don't allow connecting generic (free) Gmail accounts - only corporate email accounts.