Flashing file folder with question mark

Hello,

i have a mid-2010, 15"' MacBook Pro running Yosemite. Also, about two months ago, I replaced the two, 2 GB memory modules with two 4GB modules. Just in the past week the laptop has been acting very sluggish; login won't complete; and today, when first booting it up, I got a gray screen with a flashing "question mark" file folder. I've NEVER seen this before. In all the years I've been an Apple fan and user, I've never experienced a moments trouble.

In searching for other questions similar to mine, I read to restart in Safe Mode, select Disk Utility, then run Disk Repair on the Macintosh HD volume; exit Disk Utility, and restart from the Apple menu in regular mode. These steps worked however, once the laptop began restarting, it took at least 10 minutes to get to the desktop.

I am considering backing up critical files and wiping the hard drive clean or restoring it to factory. I don't know if this is necessary or if it would rid the system of whatever it is that's causing these issues. Also, I've not ever done this before so I don't know what issues I may encounter, or exactly what steps to take to accomplish this. If such drastic measures are not necessary, suggestions as to what I can do to correct these issues are appreciated.

Suggestions are welcomed. Thanks in advance.