Specifically what actions taken by Obama caused this?
Glad you asked
Obama saved us from a depression by stepping in and saving the banks from collapsing and shoring up our auto companies until they could get back on their feet
His economic stimulus reversed a falling jobs market and put the country back on firm economic footing
While investors were in a panic and bailing on the market, Obama assured them that the power of the US Government would stand behind our economy. The US economy, under President Obama has had the strongest economic recovery from the Great Bush Recession of 2008 of any nation in the world