restarted problem imac mid 2014

Hi all. As the title suggests I've been having some issues of late with my iMac. It's a mid 2014 model 8GB 1600 MHz DDR3. Running OS X Yosemite 10.10.5. I've had it from new for about 10 months or so. It's used a lot for work, just emails and the web. nothing heavy. The iMac started...