I came home from work and found my Early 2011 17″ MacBook Pro flat even though the charger was plugged in.
Looking closer at the charge led I found it flickering/flashing orange very quickly and the charge indicator leds on the laptop were not indicating charge.
I was unable to power on the laptop at all.
I tried another charger but this had the same result, indicating the issue was with the laptop.
After googling I found a thread on the Apple Support Communities with other people having the same issue.
As per the link below, to resolve this issue I unplugged all devices from the laptop (including power) and unscrewed the bottom case, found the battery connector and unplugged the battery. I waited 5 minutes and then plugged the battery back in and put the bottom case back on.
On testing the the charge indicator now showed an orange light, and pressing the charge led indicator button on the laptop confirmed the battery was charging.
Do not remove the battery or touch anything else on the machine, as you may void your warranty. You can use the iFixit guide below, just remember to stop after step 3, you only want to unplug the battery not remove it.
Apple Support Community thread: Won’t start up; charger blinking orange
iFixit guide: Battery replacement for 17″ Unibody MacBook Pro (should be fine for other unibody MacBook Pros)