Q: keyboard quits working

Greetings Barna101010,

Thank you for using Apple Support Communities. 

I see that you have downloaded an OS X El Capitan update. After downloading, the keyboard of your MacBook quits working, and you have to restart if you want to type. That's certainly something that I wouldn't want to happen. I want you to be able to use your keyboard. Let's see how I can help out. 

First, try to start your computer into safe mode. Safe mode (sometimes called safe boot) is a way to start up your Mac so that it performs certain checks, and prevents some software from automatically loading or opening. Once you are in safe mode, test to see if you are able to type normally, and then reboot regularly and try there. Please use the following article to start into safe mode:

OS X El Capitan: Start up in safe mode

If it's still not working correctly after safe mode, use the following article to reset the System Management Controller (SMC), and then test:

Reset the System Management Controller (SMC) on your Mac

Cheers.