Unfortunately, Google does not provide a way to recover a deleted assignment (also known as Coursework). We hope some day they'll add that capability, but at the time of this writing, they have not.

If you used the deleted assignment and students submitted work, those submissions may still be stored in Classwork.com under the old assignment. However, scores cannot be sent to the deleted Google Classroom assignment—you’ll have to re-create a new assignment in Google Classroom and manually transfer student grades if needed.

We're sorry for the inconvenience this might cause. You can use this link to request that Google add a deleted assignment recovery feature: https://support.google.com/edu/classroom/answer/10011324.