Starts but turns off immediately. red symbol lights up

That can mean the bag is full.

the red symbol probably is the temperature light.This means you motor is over heated.Check hose,wand and power head for a block...Chris

Check for a filter where the air blows out of the machine. Could be clogged.

Go to their website, they have diagnostics available to help you... http://homesupport.irobot.com/app/home.

Step on the blue lever at the base of the floor attachment to separate it from the long wand. Sometimes there is a clog inside at the base that needs to be cleared. Good luck.

Remove the battery, hold down the POWER button for ten seconds, then put the battery back in and charge overnight. If it still tells you it's out of power as soon as you start it, it's time for a new battery. http://store.irobot.com

When the bag is full, the "full bag" light will come on and will turn itself off. Replace bag (make sure there is no clog) and try again.
Hope this helps.

motor might be burnt up from the clog.

check all the filters especially the one around the motor they could be clogged with fine dust

try this: turn it upside down and run it-then bump it to reverse it...same result?