Are you trying to access the control from within the ASCX code behind, or
from within the Page's code behind?
If you're trying to access it from the Page's code behind, this should help:
http://SteveOrr.net/faq/PassDataToUserControl.aspx

Signature
I hope this helps,
Steve C. Orr, MCSD, MVP
http://SteveOrr.net
>I have a hidden control
> <input id="FieldSortOrder" type="hidden" name="FieldSortOrder"
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>
> Thanks.