MacBook Pro 13-inch Mid 2012 white flashing screen

This has happened a few times now, when I turn on my Macbook Pro the chime can be heard and I can sign in, once I have signed in the screen goes white then my desktop can be seen (no icons on the desktop are there though or the taskbar at the top) and the screen goes white then to the desktop then white again and no keys respond, I shut down my mac by holding down the power button then my Macbook works perfectly after this, any suggestions on how to sort this?