I am having the exact same problem,  tried reinstalling and tried the verify integrity of game cache option and neither has worked... I even tried reinstalling steam to no avail.  I have tried everything i know to try other than a clean install of my operating system which I am hesitant to do because one game isnt working ....

I wish Sins didnt use steam I have had trouble with steam with a number of games in the past. I hope the final release isn't only available through steam or I will not be playing it.  I dont think the Sins guys care if one person doesnt play though.

1.Completely log out of and exit steam

2.Then navigate to your steam folder and you'll see ClientRegistry.blob

3.Rename this to ClientRegistryOLD.blob

4. Restart Computer then log into steam

This will force steam to re acquire any updates that might be causing it to do that. Good Luck!

You can also trying running it straight from the EXE.

It should be located in,

C:\Program Files (x86)\Steam\steamapps\common\sins of a solar empire rebellion 

However you may have it labeled as another Drive or moved all of your games to an external / SSD.

Also you mentioned not being tech savvy no problem, it should say "Sins of a Solar Empire Rebellion" , it's type is application and it's size is around 5 MB  if you are looking at the columns next to it.

If there's no exe file and you've installed it twice, check your antivirus logs. Most likely that's the guilty party.

It worked. Mucho Gracias.

Nice necro...