no bootable devie

I turn my computer on today and got the error no bootable device- insert boot disk and press any key - then i turned it off and tried again and it started right up, my question what would cause this and is it a sign that computer is on its way out

Bill

Model Name: iMac

  Model Identifier: iMac14,2

  Processor Name: Quad-Core Intel Core i5

  Processor Speed: 3.4 GHz

  Number of Processors: 1

  Total Number of Cores: 4

  L2 Cache (per Core): 256 KB

  L3 Cache: 6 MB

  Memory: 8 GB

  Boot ROM Version: 146.0.0.0.0

  SMC Version (system): 2.15f7

  Serial Number (system): C02L94ZRF8J5