Need help on start-up, grey screen, after a while the pane for utility disc etc. comes up. I used utility disc and it said appears to be OK then goes to restart and starts ok. Next time it says needs repair then can’t be repaired. Then appears OK an

Need help on start-up, grey screen, after a while the pane for utility disc etc. comes up. I used utility disc and it said appears to be OK then goes to restart and starts ok. Next time it says needs repair then can’t be repaired. Then appears OK and starts. I noticed the start-up disc (hard drive) has a question mark ? I selected it and restarted and it’s still there. Should I reinstall Mountain Lion 10.8.5 ? it has been working great up to now.