Getting error in activation email in reg page

im getting error in .....


private string Fetchsno(string emailid)
{

}


Error: '_Default.Fetchsno(string)': not all code paths return a value


I want to use this funtion activation url.....
suppose if i have used void datatype means,

im getting error in activation url...

ActivationUrl = Server.HtmlEncode("http://localhost:50059/WebSite1/activate%20account.aspx?sno=" + Fetchsno(emailid) + "&emailid=" + emailid);


error:

Error   16  Operator '+' cannot be applied to operands of type 'string' and 'void'  D:\frm lap\WebSite1\Default.aspx.cs 33  44  D:\frm lap\WebSite1\

Give me your suggestions