prohibit/no entry symbol upon rebooting my computer
Hi there, 3 days ago I saw the prohibit/no entry symbol upon rebooting my computer. The computer started up with on its own however, it was very slow and stalled multiple times. 2 days later I attempted to install updates to the computer and upon the reboot I was once again greeted with the same symbol, this time there was also a flashing folder symbol at times with a question mark. I was unable to restart the computer using all of these articles:
A flashing question mark or globe appears when you start your Mac - Apple Support
Mac OS X 10.2.1: Kernel Panic After Updating to 10.2.1 or Optimizing Mac OS Standard Volume
Mac OS X: Gray screen appears during startup - Apple Support
The only results I got where when I help down the option key and used disk utility, where I attempted to use the first aid tab (as directed in the first article) to which it found a volume issued, it "repaired" it and I shut down only to get back to square one. After going through this process and various start ups for multiple hours I left the computer for a day. I then turned it on today with no issue (the same lack of speed and stalling have greeted me). My question itself is it how to go about fixing this problem so that I may continue to use the computer as normal and it returns to its full operational speed, as I am quite fearful of shutting it down again. Help would be greatly appreciated.
Details:
MacBook Pro purchased in 2012 (no OS X installation disc or drive or any kind)
Currently using Yosemite 10.10.2