How to disable button on ascx page from aspx page

I have a button on a web control page(.ascx) that I need to disable from an aspx page (depending on certain criteria).  I just need to know how to call that control(the button) that is on the .ascx page, from my aspx page (in the page load event for example). 

I think I have to use the 'FindControl()' function but I am not sure.  Thank you for any help!