After Postback the fields blank

The answer to your question is very simple. Anything which don't have runat="server" will not persist after postback. If you put runat="server", but want the client Id doesn't change than use the clientidmode="Static" and make sure no two ids are same.

If you have runat server attribute set, then Sandeep's answer explains why. You may be disabled viewstate or you initialize the textbox in the page_load to an empty string without a check for postback.

Explain more if you have still issues.

hi visit this link

http://forums.devshed.com/net-development-87/how-do-i-keep-a-variables-value-during-postback-119008.html

In your case looks like:
the code that fills the text box via javascript needs to be attached with onload event of page at script side
like:
try this in your javascript

<br />
<br />
function fillTextBox(){<br />
// fill your text box here.<br />
}<br />
<br />
window.attachEvent('onload', fillTextBox);<br />
<br />