connection string reading data from excel in asp.net

The issue here is that for this sort of 'file' connection, the OleDb provider needs, as you've already found, the absolute path to the file. Historically, we've done this with Server.MapPath, in which case you'd do something like:

<connectionstrings>
    <add name="xls" connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Extended Properties=Excel 8.0;"/>
</connectionstrings>

String connString;
OleDbConnection conn;

connString = Web.Configuration.WebConfigurationManager.ConnectionStrings["xls"].ConnectionString;
connString = connString + Server.MapPath("~/App_Data/HPM_DB.xls");

conn = new OleDbConnection(connString);

conn.Open();
...

which works, but you need to either do all that every time you want to open a connection and query the data, or break it out into a connection factory class.

A neater way to do it is this:

<connectionstrings>
    <add name="xls" connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Extended Properties=Excel 8.0;
    Data Source=|DataDirectory|\HPM_DB.xls;"/>
</connectionstrings>

OleDbConnection conn;

conn = new OleDbConnection(Web.Configuration.WebConfigurationManager.ConnectionStrings["xls"].ConnectionString);
conn.Open();
...

At runtime the |DataDirectory| token in the connection string acts as a macro which silently does all that Server.MapPath stuff for you.