Prohibit/no entry symbol upon rebooting my computer

Question

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

Answer 1

Reinstalling OS X Without Erasing the Drive
Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Reinstalling OS X Without Erasing the Drive
Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.
When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the Macintosh HD entry from the the left side list.  Click on the First Aid tab, then click on the Repair Disk button. If Disk Utility reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit Disk Utility and return to the main menu.
Reinstall OS X: Select Reinstall OS X and click on the Continue button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.
Alternatively, see:
Reinstall OS X Without Erasing the Drive
Choose the version you have installed now:
OS X Yosemite- Reinstall OS X
     Note: You will need an active Internet connection. I suggest using Ethernet
                 if possible because it is three times faster than wireless.