Enter Key Stroke Will Not Removed All The TEXTBOX Data

The WEBFORM1.ASPX is working well and I am very Puzzle by the data entry TEXTBOX. Each time after the User has input the data and hit ENTER KEY, very strangely all the TEXTBOX contained data have been cleared off data. How do I ensure that when ENTER KEY is pressed by user it will not removed all the TEXTBOX Data. Here is the sample coding of the data entry TEXTBOX.:

<div class="LeftMargin">
<table style="width:100%;"
<!--&nbsp;------CustomerID&nbsp;and&nbsp;Address&nbsp;------&nbsp;-->
<tr>
<td align=left class="style2">
<asp:Label ID="lblCustID" CssClass="labelText" runat="server" Width="160px" Text="Customer ID: *"></asp:Label>
<asp:TextBox ID="txtCustID" CssClass="textbox" runat="server" ></asp:TextBox>
</td>
<td>
<asp:Label ID="lblCustAddr" CssClass="labelText" runat="server" Width="150px" Text="Address : *"></asp:Label>
<asp:TextBox ID="txtCustAddr" CssClass="textbox" Width="250px" runat="server" ></asp:TextBox >
</td>
</tr>
<!--&nbsp;------&nbsp;Contact&nbsp;name&nbsp;and&nbsp;City&nbsp;------&nbsp;-->
<tr>
<td align=left class="style3">
<asp:Label ID="lblContname" CssClass="labelText" runat="server" Width="160px" Text="Contact Name :*"></asp:Label>
<asp:TextBox ID="txtContName" CssClass="textbox" Width="300px" runat="server" ></asp:TextBox>
</td>
<td class="style1">
<asp:Label ID="lblCity" CssClass="labelText" runat="server" Width="150px" Text="City : *"></asp:Label>
<asp:TextBox ID="txtCity" CssClass="textbox" Width="250px" runat="server" ></asp:TextBox>
</td>
/tr>
<!--&nbsp;------&nbsp;Only&nbsp;PostCode&nbsp;------&nbsp;-->
<tr>
<td align=left class="style2">
<asp:Label ID="lblBlank1" CssClass="labelText" runat="server" Width="460px" Text="" ></asp:Label> </td>
<td>
<asp:Label ID="lblPostCode" CssClass="labelText" runat="server" Width="150px" Text="Post Code: * "></asp:Label>
<asp:TextBox ID="txtPostCode" CssClass="textbox" Width="250px" runat="server" ></asp:TextBox>
</td>
</tr>
</table>
</div>