wont start

i have a 1991 s-10 pickup and it wont start. It turns over, it has all the fluids, fuel pump works, gets gas to the carberater. what could it be. it wants to crank but something wont let it. you can hold the key forever and it still wont crank.

what size engine? check for spark.

You lost me , When you turn the key does the starter crank over the engine ??? or does nothing at all??