no electrical

I just recently purchased this truck (Triton). I was driving to work and the oil light came on and truck shut off. i pulled over and noticed that the oil was fine tried to restart truck and nothing. it has a good battery, but the truck was having a hard time starting since i bought it. it acted as if the starter was week. i pulled it to work and a co worker tried to turn the motor with a ratchet and said it wouldn't turn and it might be locked up. any ideas? because i can't seem to think it could be locked up.