You'll need to connect your email account once again using big red plus button on Settings -> Email accounts page.
- Once it's done, make it Default as shown on the picture below.
- Remove the disconnected (red) email account.
- Connect the Gmail account via Single Click again by using the red plus button.
- Go to your campaign -> settings and re-assign email account for sending.
The campaign will be resumed within 10-15 minutes according to your Schedule.
P.S. If you already have full email accounts quota reached, and your disconnected email account (red one) is Default -> make another one Default -> remove the red one and then connect it again as described above.