Hi, are there any error messages? The most common cause of this is that the player has selected a race in a mod that does not exist in the main game, and cannot be loaded. However, the latest version (1.37) will now flip back to the federation in this case. There may be some ship designs left over which are referencing content that is no longer installed.
The easiest way to fix this is just to rename this folder:
\mydocuments\my games\gratuitous space battles
And then restart the game.
If you do plan to conduct another reinstall of the game
As a suggestion, delete the install location first and rename the GSB directory in My Documents\My Games\GratuitousSpaceBattles (as suggested above)
That will get the game working in 99.9% of all situations.