keep getting a drop-down menu telling me there is a problem and I need to power off my laptop and restart. It doesn't always come back on when I try to restart it.

Have been getting a drop-down message that tells me I need to power off my computer and restart. Happened yesterday for the first time and 5 times so far today. Three of those times I could not get the computer to come back. Could hear the musical sound as if it was turning on but screen was blank. Just had this thing in last week for a checkup at the Apple store and everything checked out perfect!