Unable to boot normally on samsung S5

My phone shut itself off for a few hours, would not charge or turn on, came back on briefly after about 4 hours, then shut itself off again, again would not charge.  I tried a different battery...nothing.  I cleaned around the battery prong connections.  It came on with an odd message, saying unable to boot normally, some code, a large green android man icon in the center of the screen, and a message below that saying Downloading....do not turn off target.  I let it sit that way for a length of time and nothing happened.  Shut back off.  Any suggestions?