function plpy() {
var x = document.getElementsById("videox");
if (x.paused) {
x.play();
}
else {
x.pause();
}
}
<asp:toolkitscriptmanager id="ToolkitScriptManager1" runat="server" xmlns:asp="#unknown">
<asp:modalpopupextender id="ModalPopupExtender1" runat="server" backgroundcssclass="modalBackground" targetcontrolid="B" popupcontrolid="Bsong" xmlns:asp="#unknown">
<asp:button id="btnplay" text="Play/Pause" onclientclick="plpy()" xmlns:asp="#unknown">
| <asp:button id="btnrest" text="Restart" xmlns:asp="#unknown">
| <asp:button runat="server" id="imgcloseB" text="Close" xmlns:asp="#unknown">
|