How come my 87 Mazda b2000 won't crank over?

1st is the battery showing at least 12.5 volts not running.Now with a multimeter you can check this.If the truck has a fully charged battery,and the alternator was good before the issue,next question is do you here a clicking sound when trying to start the truck.If you answered no click,the issue is probably the starter is bad,or the armeture inside is stuck.You can simple tap the starter with a flat head screw driver and hammer a couple times to free it inside.Then retry starting the truck.If the trk does not turn over or say click and the voltage is GOOD as above and the battery is fully charged then you need a new starter.Sometimes tapping the starter will make it engage and work but,if this happens it normally means the starter is dieing and replacement is a MUST.Now if the truck is turning over and not firing up and running I would look at checking the ohms on the coiland should be 1-1.3 ohms.And If the part is original probably needs to be replaced.It could also be the distributor cap,or rotor button are bad or the ignition module.There are too many things to list like bad plugs,or spark plug wires which would cause a no fire but,engine turns over problem.I would charge the battery for 24 hrs and do the checks above if it is a no crank at all,bc it could be your alternator may not be charging the battery enuff to start the engine to begin with and it should show at 12.0 volts to start as a minimum but most should show on average 12.3 volts with the trk turned off