iMac doesn't start easily sometimes
When starting up my iMac (27inch, late 2012, Mountain Lion,1Tb Fusion disk) in the morning, after cooled off during night, sometimes I have to try 3-4 times before it works out. The apple sign appears, I can can hear the disk starting, and then nothing happens. The frequency of this event is increasing so I am starting to worry that one of these days it won't start at all. Anything I can/should do?
iMac (27-inch, Late 2012), OS X Mountain Lion (10.8.5)