Far as I'm concerned, using the document.getElementById function requires the ClientId as parameter, and not the name. ctl00$ContentPlaceHolder1$btnSubmit looks more like a name, the Id would probably be ctl00_ContentPlaceHolder1_btnSubmit. To avoid confusion (and really, not try to guess what a ClientId is going to be I would use:
[...]
onkeydown="OnTextChange('<%=btnSubmit.ClientID%>')"
[...]
var question = (_2b || !(_2b));