You need to go into steam, then find the game in your library, right click and choose properties. Then choose local files, then verify game cache. That should fix it.

it failed to validate how caan i fix that

AAAAAAAAAAAAAAAAAAAAAAAAAAAAHHHHHHHHHHH IM AT THE EDGE IV TRYED EVERY THING EXEPT UPDATING PORTAL 2 BUT I CANT FIGUER OUT HOW TO DO IT IT DOESNT DO IT AUTOMATICLY EVEN THOUGH IT SHOUD IV WAITED SO LONG FOR THIS GAME AND I FINALLY GET IT AND I CANT EVEN USE IT!!!!! IV ALREADY TRYED VALIDATING AND IT FAILS EVERY TIME IM F*CKING P*SSED OFF AT STEAM RIGHT NOW YOU DONT EVEN KNOW JUST HURRY UP AND HELP ME!!!!! IM READY TO SUE AND IF IT DOESNT WORK ILL FIND A WAY TO GET MY MONEY BACK FOR THIS PEICE OF CRAP. KEEP IN MIND I STILL WANT THIS GAME I JUST DONT WANT TO BUY A BRAND NEW CONSOL TO GET IT ALL I HAVE IS A LAPTOP. JUST HELP ME!!!!!!!!!!

You can try reinstalling. You won't lose your custom test chambers (at least, not the ones done in the puzzle maker, not sure about hammer).

If you go to your Library, right click on Portal 2 and select "Delete Local Content...". Let it run and when it's done, you should see an "Install" in place of the "Play" button. Reinstall and see if that fixes the problem.

it didnt