Does the engine rev up very high but the vehicle does move? Sort of like if you had the clutch in on a manual transmission and the power doesn't go to the wheels? If yes the transmission is shot. Does it not rev up at all and stutters or doesn't move? You've got at the very least a Pending Check Engine Light that you can pull with a code reader to get some clues as to what is wrong.

Even with busted plugs with no center electrode the engine will still spark and move so there's something much more serious going on either to the engine or transmission. However we need more information in order to help, just saying it won't move faster than 15MPH doesn't give us anything to go off of.