My MacBook Pro shut of three times yesterday after plugging my phone in but I managed to switch it back on each time. I left it to rest for about 20 hours and then today it Has been working fine

My MacBook Pro shut off three times yesterday after plugging my phone in but I managed to switch it back on each time. I left it to rest for about 20 hours and then today it Has been working fine all morning. Using indesign and Skype which usually slow it down anyway but it's been fine. Then I plugged my iPhone 4S into it and it just shut off again. But this time it won't switch back on. No idea why this has happened but I hadn't saved my work either. I'd not long plugged the charger in to it but now, it's still plugged in, but the light on it is very dim and is just flashing.

No idea what to do. Please help!