We are having an ongoing problem with our Inspiron 15/3520, as follows:

• Over the past few months, there have been about 7 instances of getting the 'Bitlocker' screen come up. Fortunately, we have managed to get the code and recover the situation.
• In the past two days, we have had a problem in starting the laptop. Yesterday, after pressing the Power button, nothing happened although the mouse was visible (and movable) on the screen. After a long while, the laptop booted. Today, same behaviour, except that after a while, it went into the Bitlocker screen again.

Anyone have any ideas as to what is causing this?