Automatic shutdown and starts giving 7 beeps

Sir,
I have a dell inspiron 5520 laptop. It was working properly since two years but due to accident overheating occur may be since message appears " power surge on the hub port" and its all usb ports stop working and then i made it repaired and got two of its ports worked well after just one week problem occur that it get automatically shutdown and when i turn on power button starts beeping 7 times. And after 2 or 3 hours it again started normally and get shutdown again and continuously this is happening. Plz help me i dont know what shud i do??