I lost my *.rsd file for my site but the site is currently live.

Is there any way to open up the exported site in Responsive Site Designer to edit it?

I am sorry, but no. If you lost the project file, you will have to re-create your website over again.

Well that's 50 pages of wasted time....

A great reason to make backups!

It may be worth considering whether the RSD file is irretrievably lost. If the hard drive on which it used to reside hasn't been physically lost, there are various programs that may be able to recover the file if it hasn't been overwritten.

For example, a friend had formatted an external drive which he used with Windows, but subsequently discovered that he needed some of the files on it. I managed to recover them with Zero Assumption Recovery (http://www.z-a-recovery.com), but that is just one of many programs that will attempt such recovery.

Frank