Unless your USB drive is bootable, as in you took special steps to make it bootable, you won't be able to boot from it.

Run Dell Diagnostics at bootup F12.

I suspect you have a failing/failed hard drive.

Update:

The test for hard drive passed:

**Hard Drive - DST Short Test**

Test results : Pass

but while testing for memory it suddenly crashed. I noted that it was testing for March B test when it crashed. I dont know what it means.

Thanks for your reply,

No I didnt made USB bootable. When I open the BIOS window (Pressing F2) I can there my hard drive mentioned. It also shows the disk size. Does that mean my hard drive is ok ?

I ran Diagnostics. While testing for Hard drive, it didnt said pass. It said "running" and went on next test. 

Try running the Hard Drive Long Test

if the HDD test passed then it is probably just a matter of reinstalling Windows. However, if the computer shut itself off during diagnostics it might be overheating or have a fan/overheating and/or motherboard issue.