If you installed Raspbian from the NOOBS distribution as I do, then you may occasionally “blow up” your installation as I just have! You have an out, sort of, short of re-imaging the disk, though about with the same impact.
To interrupt the boot process and enter recovery mode, attach a USB keyboard and repeatedly hit the Shift key. You should come to the NOOBS OS install selection screen. Just re-install Rasbian again… But if yuo’re using WiFi first configure your WiFi setup before re-installing Raspbian.
When I powered up, I got the initial multi-color screen. Then a two-line text message popped up – too quickly to be read, then a grayish screen, then it split into a lower and upper part, then both halves faded away and there it stayed… At that point it was not responsive to any keyboard inputs or mouse clicks.
While doing my advanced slide show and rotating display project, I somehow managed to blow up my OS. finding the way to interrupt the boot-up was not so easy so I am amplifying the answer that worked for me on the Internet: repeatedly hit the Shift key during the boot, until you see the NOOBS image selector screen.