When your app can't run and can't figure out why.

logcat does not show a trace?
widen the filter to see traces from the system as well.

Check for nullpointerexceptions maybe

@dontPanic Sorry, I'm A noob at java can u explain a little further ?

@moghir2004 look at your stacktrace (the box on the bottom of android studio) and check whether there are any signs of NullPointerException. It's a common error in Java and it comes to live when you try to access a null object reference

Logcat is your friend! It often even shows you which line is wrong.(The blue, underlined. Not the gray.)
Or just paste the firstline of the error into google. It's often a common error.