Acer Aspire One dies when power disconnect

Hello,

Please please please can somebody help?
My netbook works fine when it's connected to mains power but it's started crashing when it's running on the battery. Anything from 2 seconds to 10 minutes from disconnecting the power the screen goes blank and the hard drive light goes out but I can still hear the motor(?!) running inside.
I would really appreciate it if somebody could help me out because I'm not very good with computers and I need this computer for my new job.
Thankyou so much.