Bad Interpretor

I have installed cygwin in my windows machine. when i ran the sample script it was throwing the following error.
bash: sample.sh: /usr/bin/bash: bad interpreter: No such file or directory Till yesterday it was working fine. I dont know all of a sudden what happend.
i used to run my script by simply typing the script name. Now it is working only if I type sh <script name>. What went wrong? i have added C:\cygwin\bin to path variable.
Any help is appreciated.
Thanks in advance.

sample.sh

#!/usr/bin/bash
echo "Hello world'

output of which bash: /bin/bash