My Nikon D3000 no longer auto-focuses at all?

A few days ago i attached one of my old Sigma legs for my Nikon N65 film SLR to my D3000. My digital SLR still worked fine and was able to take photos however when I put my digital SLR lens back on my digital camera the auto focus no longer works. Is this because the setting on the camera changed automatically when I changed to an old film lens? If so, how do I return my digital SLR to its original settings that are compatible with the digital lens?

More likely you moved the A/M switch on the lens to the M position

Either that or the lens contacts are dirty. Pull the lens and gently clean the gold contacts. That's happened to me. Even the slightest bit of dirt can stop contact between body and lens.