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.