i can't launch the stage

[HugoCastroBr]

I have a problem, when i will launch the first stage/my rocket, i press space but not happen i can't launch the stage, the stage does not change, pls help me

[Popestar]

If you are using liquid fuel engines - and I assume you are because you haven't given much to go on - you have to throttle up to generate thrust.  Throttle to max with Z, or little by little with SHIFT.  Cut throttle completely with X, or little by little with CTRL.

[VoidSquid]

Hi there and welcome to the forum, @HugoCastroBr !

To be able to help you, we need a bit more information as outlined here:

 

and here, respectively (unmodded / modded installs)

 

 

This particular issue you're mentioning, I've seen once, turned out a Steam file verification could resolve it, why not give that a try.  

[king of nowhere]

check if your liquid fuel engines are throttled on. check if you have control (an astronaut on board, or a probe core with electricity). try to press space again. try to restart the game.

if all this fails, take some screenshots and post them here.

[Daedalus3000]

Did you by and chance hit Mod + L? This causes the stage light to turn pink and prevents you from staging.  (To fix this just hit Mod + L again). If the stage light is green than you probably just forgot to throttle or it is a bug.