Back Screen on Reboot?

2.6GHz Mac mini

• 2.6GHz dual-core Intel Core i5
• 8GB memory
• 1TB hard drive
• Intel Iris Graphics
• OS X Yosemite 10.10.3

So I got this Mac a couple of months ago and ever since I have had it occasionally when I reboot it, normally when installing updates it doesn't reboot properly and just sits there with a black screen and no cursor.

I have to hold down the power button to turn the Mac off then turn it on again. I've had to do this about a dozen times since having it and this cant be good for my HDD.

Does anyone have any idea why this is happening? This is my first ever mac and I was expecting it to be a bit more stable than this?

Thanks in advance,

KB.