I hope you want to insert the textbox value.so modify your query like this
string cmdSendSMS2 = "insert into MobileSMS(MsgTxt,MsgDate) values
('" + msgtxt.Text + "','" + DateTime.Now + "')";
where your textbox can be like this
<asp:textbox id="msgtxt" text="" runat="server" xmlns:asp="#unknown"></asp:textbox>
Then it will insert with the value provided in the textbox