Did you get part-way through the import process and now can't progress? Have you noticed any warning messages on screen (for example 'file invalid'?).
'File is invalid' will display when there are characters in one or some of your fields that commonly are used in programming languages. Common characters that are known to cause issues on occasion include:
Quotation marks: "
Parentheses: ( )
Carriage returns: these are a little tricky but you can generally view them in a text editor such as Notepad or Notepad++
Another issue that can cause your import to either fail to start or fail to finish is a significant number of additional 'blank' fields in your file - these are also easily visible when viewing your data file in a text editor such as Notepad or Notepad++
Besides 'invalid' characters, it could be that the file you're trying to import could be a format that isn't supported by gCast. gCast supports csv and tab file formats for uploading your members. It is important that your csv file is actually comma delimited (not tab) and that your tab file is actually tab delimited (we quite often see a file that is named as a .csv file but when it is viewed in a text editor, the delimiters are actually tabs).