1 beep and doesn't boot

I have a 15 inch Macbook, old one. I have this problem. I didnt do any updates or anything, just like any time, I closed the lid and now when I opened it the screen is blank. I tried the power switch, and the mac beeps with the light flashing [1 beep then a pause, and again 1 beep, pauses and it goes on ]

I tried powering on and then pressing and holding shift key, it works, the mac boots and I can work on it. But again when I reboot or start it after shutting down, the beep comes and then again I have to power on with the shift key.

I tried the sbin/fsck -fy and it did the startup checks and made some fixes but still the problem persists. Please help me.