Oracle Update got problems

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ADO.NET

Oracle

I created an Oracle Data Table either in Oracle SQL Developer or in my .Net program, and also OK in updating. Today, I got something strange, as summarized below:
In Oracle SQL Developer
1) Delete tbl_xxx - working to delete all of the records
2) Truncate Table tbl_xxx - Not working, got frozen
3) Drop table, Edit (e.g. add a new column, or change table name) - Not working
In .Net Code
1) Delete tbl_xxx - Not working when running

try {
using (OracleCommand cmd = new OracleCommand("delete " + tblName, connection)) {
    cmd.ExecuteNonQuery(); 
}
catch (System.IO.IOException e)  {
    g.Write2Log("Error Message: " + e.Message);
}

Got frozen at cmd.ExecuteNonQuery(), but no error message was thrown.
I did another test:
1) In Oracle SQL Developer, made a copy of the data table.
2) Could perform all of possible functions, like drop, truncate, edit,etc.

I learned that the table, which got problem as stated above, is currently used in a webservice. I am not sure this is could be the reason to cause the scheme lock or not.

What could be the problem be possibly solved? How can this be debugged w/ error message? Thanks in advance if you can share your experience.