Mac doesn't start correctly
When I start the mac it doesn't work, and, instead of starting, a logo of a folder with an interrogation sign (?) on it appears. I have to forcibly switch off the computer and switch it on again, sometimes several times, until it works and finally starts correctly and I can login. What can it be? Has anyone experienced the same issue? Thanks!
MacBook Air (13-inch Mid 2011), OS X El Capitan (10.11.4)