OK, the first thing to notice is that DateTime.Now is a very specific time: it is literally the moment in time when you access the property. And when you subtract 24 hours from that, you get the same exact time, but yesterday. Since this time is accurate to milliseconds or less, it is very, very unlikely that your query will ever work, unless you are lucky and posts are made at the exact same time each day!
So, start by looking at
DateTime.Now.Date[
^] which discards the time part and goes back to midnight.
Then change your query to look at values greater than that:
string strQuery = "Select Item_Purchased_Number,ProductID,ProductName,quantity,Branch, Date_Purchased from Customer_Purchased Where Branch = 'Manila'" + "'AND Date_Purchased >= '" + DateTime.Now.Date.ToString();
(or better still, use a parameterised query and don't convert to string - this can cause problems in some systems, since the string format can be very different depending on the PC system settings).
If you want specific days results, then the way I do it is to use the SQL DATEDIFF function:
...WHERE DATEDIFF(d, Date_Purchased, " + DateTime.Now.ToString() + ")=0
Will find today, "=1" will be yesterday, and so forth.