MacBook automatically sleeping at login screen

Hi,

I've gone to use my MacBook today and it seems to have developed a fault. It isn't automatically waking from sleep when I open the lid and if I restart it it outs itself back to sleep as soon as it gets to the login screen.

I'm guessing it's something to do with a sensor that registers when the kud us opened? Any advice?

Thanks

ok, I've connected it to an external display which seems to work fine, however when it is connect it doesn't seem to recognise it's own display. This is working as when the computer boots up it displays the apple logo and very briefly the login screen before putting itself to sleep (unless connected to an external display).