Snowblower sheer pin for one blade is gone but the cutter won't spin so i can't line up the holes to replace sheer pin

Look close to the middle of the auger. you should see small ( 1/4") holes where the shear pins were. Just line up the holes from shaft to Auger and put new Shear pins in there. Sometimes they look like Clevis pins and sometimes they look like a simple 1/4" bolt and nut

Shear pin (connects the blade to the shaft ) is broken.

You can certainly use a 'compression tester", like mechanics use on car/truck engines. They're not really expensive, but the test will take two people, one to pull the starter cord, one to hold the tester to the sparkplug hole.

With the motor OFF, the shaft shouldn't rotate, so you should be able to rotate the auger by hand until the holes line up.

Check to see if cable is still ok and not broken. Also adjusted.

To change the head on the trimmer you will need to turn the trimmer upside down as if you were going to take the head off to restring it, spin the head and look for a slot in the side of the head when you see the slot, keep spinning until you see it line up with a hole in the metal piece the head attaches to. Stick a nail in the slot and hole and turn the head off the trimmer, if I remember correctly the head is backward thread.
Follow the steps backward for putting the brush blade head on the trimmer.

Yes there are, usually what is called a sheer pin. They are designed to break before anything else is broken. They are found on the auger shaft. You probably can't see them because they are broken and have fallen out. Inspect the auger shaft where you should find holes where they go. Go to a shop or where they sell parts and get the proper sheer pins. If you can't find any, ordinary bolts will work just as well. Install them in the holes you have found on the shaft (I hope). If the bolts are still there on the auger shaft, then there in something else wrong obviously. You say everything looks ok, so that is surely the problem. I hope this helps you even just a bit. Best regards and good luck.

the augers on many craftsman snowblowers have a sheer bolt. If the auger encounters too much resistance, ie, a foreign object like a piece of wood or even a newspaper buried in the snow, this bolt will break off preventing damage to the gear box, which is a lot more expensive to repair than replacing the bolt.
This bolt requires not tools to install, just insert and secure with a cotter pin.
The parts dept at sears can sell you the replacement bolts with the correct sheer force rating. Have your snow blower model number handy when you call or go in.

You may have sheered the key way off on the outputshaft. Remove the blade and inspect.

You need what they call a "friction wheel" replaced.