Hi makumbi,

For atleast one checkbox checked in gridview refer below article.

Validate GridView with CheckBox (at least one checked) using JavaScript in ASP.Net

Refer below code for TextBox validation.

HTML

<asp:TextBox ID="txtemail" runat="server" Width="168px"></asp:TextBox>
<asp:Button Text="Send Email" runat="server" OnClick="OnSendEmail" Height="26px" ID="btnSubmit" OnClientClick="return ValidateEmail()" />
<script>
    function ValidateEmail() {
        var email = document.getElementById('txtemail');
        if (email.value === "") {
            alert('enter the email address');
            email.focus();
            return false;
        } else {
            return true;
        }
    }
</script>

Screenshot