first check with file whether it open or not
if it open close it and build the application eve though your facing the same try with the following code
These kind of issues may occur when "SNo"column may contains the alpha-numeric values.. try with the below code where convertion happen's
connectionString =
"Provider=Microsoft.Jet.OLEDB.4.0;Data Source="
+ fileLocation +
@";Extended Properties="
+ Convert.ToChar(34).ToString() +
@"Excel 8.0;Imex=1;HDR=Yes;"
+ Convert.ToChar(34).ToString();