Laptop shuts down automatically after being in the sleep mode for a while.

Since last quite a few days I am noticing this problem, after not using the laptop for a while or when the lid is closed, the system goes in sleep mode as it should. However, it doesn't remain in sleep mode for long. After a while the system shuts down automatically. And then when I turn on the laptop by pressing the power button, the system restarts but shows the status "Locked", and on unlocking I find everything as it was left off.
So is this a problem or some kind of defense-mechanism of the system?
Any ideas about why this happens or how to solve it?
My system is inspiron 15 3521 and runs on Windows 8.1 64 bit.