Mushromhead,
"i went to repair shop and they said there might be problem with key since on my dashboard the key sign light is blinking and doesnot stop. however key is working fine, i mean the remote"

You went to repair shop by yourself and explain the situation right. But you did not towed your car to repair shop right. If you have already towed your car to mecahnic and mechanic is just assuming this and that might be problem then let him do trouble shooting. he will come up with solution.

If you have not towed yet. and also you said you try every thing But we dont know what you tried. Please let us know what you tried.

"a friend of mine left its headlight on for 10 hrs" seems like this is the show topper.

As dekchi said try honking. does it honks loud or weak ? or you can turn your head light. will it turn on? if it turns on how bright it is or does it go dim when you crank while head light is on ? what sound you hear during cranking ? do you hear click sound? Have you check cable which is attached to battery ? is it loosly connected ?

more likely it is battery problem.
if not then may be cable which is connected from battery to starter.
if not cable then may be key area where you insert key and crank it. it may not be connecting properly so it is not cranking.
if every thing is functional mentioned above then you can check FUSE also some time this tiny things give problem and we overlook it.( i dont know ow to check it though)
Then may be there can be problem with starter( if you hear click sound but car wont start)
There can be alternator problem if car starts ( by jump start ) but cannot charge battery.

Solutions:
If it is a battery problem then try jump starting ( check proper way to do it. search in google you will see lot of video regarding this)
if jump start doesnot fix this then change another battery and try it.
If battery is not problem then my friend you have to tow your car to mechanis and they will take care of it.