If you are importing an Excel file (.xlsx or .xls), this may be resolved by saving it as a CSV. If you are getting the same error then please open the CSV file using a text editor such as notepad. It is possible that there are very many empty rows in the file which is why you are getting this error message.
The other reason is that your machine has run out of memory when you run this operation. This is not normally the case and you would probably recognise that the computer is running slowly but it is not out of the question.
If you are hosted by Blackbaud then there may be other considerations.