I wish they would create a downloader again, I would love to play Gal Civ 2 again.... oh well *sigh*
Go to https://store.stardock.com/
and log in there, using the same email address that you logged into the app with (and I believe that it's also the same email address that we get onto the forums here with, as I know that you can access this exact forum on the Stardock site).
From there, go to My Account, then Downloads. All of your legally purchased -Stardock- games are available for download there.
Anything else, such as EA games, have either been transferred elsewhere (Origin in the case of EA), or you'll just have to try and get the app running so you can download them. That's pretty much the only way I can play the Dawn of War 1 collection without purchasing the whole damn thing over again on Steam.
Hope that helps.