Could I ask which browser you are using. I have been having the same issue with Firefox and to a much lessor degree Chrome.
With Chrome, if I refresh the browser then the button will be active again.
I use IE much less, but I haven't been having the button issue with it at all. ( of course I'm on a MS operating system too) :S
Try the refresh if that doesn't work then try a different browser. Double check your system is up-to-date.
Please specifiy your issue in detail so that we can assist you better.
Which browser and OS you are using ?