There are several reasons why your DocuSign connection stopped working. One could be that the mode was switched to production (or viceversa to Sandbox mode) and there is no Account connected to the mode you have selected!
When changing the mode, you have to connect the corresponding DocuSign account again!
When you are still logged in to your DocuSign account, your browser automatically connects to the corresponding DocuSign account without redirecting to the DocuSign login page.
If you want change the connected account, please adhere to the following steps:
Log out from all your DocuSign accounts
Go to Settings > Electronic Signature and switch mode & save
Change mode again to previous mode & save
Click on ‘Link DocuSign account’ button
Log in to the account you want to use for the corresponding mode