We recommend that you always use the expiry option instead of deactivating a survey. If you closed your survey accidentally, it is important that you don't change anything in the survey!
Then, and only then, will the following steps will work:
Activate your survey again
Select Responses from the survey toolbar.
Click Import and select the "Import answers from a deactivated survey table" menu entry.
Choose your source table.
Click Import located on the upper-right part of the screen.