Stuck on apple logo at start up Mac Fix macbook pro, imac, mac mini, macbook retina display

mac pro\r
Fix to Macbook air stuck on apple logo , wont start up\r
Macbook wont boot\r
how to fix\r
Recovery System OS\r
Mac fix\r
Time Machine recovery\r
\r
macbook wont boot stuck on apple logo macbook pro wont star up \r
\r
Apple computer