can't tell direct answer to your question with these information. you can find the solution by your self.
put break point just after below line and run the program
string cmd = "select * from Login where Institute='"+DropDownList1.Text+"'And Username='"+TextBox2.Text+"' and Password='"+TextBox3.Text+"'";
check the value of cmd you get when you debug. do you have values you expect as institute, user name and password? if not you need to find why you not getting expected value.
if all these values are correct then you can copy the sql statement and run this directly on sql server management studio and confirm whether you have records for generated sql or not.
hope this helps you to find where is the issue and then you will able solve by user self or update the question with more specific details.
below are few side notes to improve your code:
1.
use sql parameters[
^]
2.
dispose the connection with using block