Our Grand Cherokee was running fine three days ago. Yesterday it would not start, will not even crank over. Battery tested good. I looked at some of the fuses, didn't find any blown. Any ideas?

no engine stated, impossible to help you, unless guessing is cool?

Gas engines 101.
so cranks robustly?
yes,
ever get a full tune up this decade. (when ?/ miles ago)
does the scan tool show errors cranking for 5 seconds.
yes, BINGO.
50 possible causes, even bad fuel.
or 5 to 10 tests.
you do 2 things, guess or test. your call.

Could be a faulty starter or bad connection between the battery and starter. You could use a meter to check for voltage at the starter but it will not show resistance in the cables.

Get the trouble code that is stored in the computer.This can get us in the right direction.

If it was running yesterday and only cranks this morning you need to determine which you are missing - spark or fuel. The spark is easy - pull a plug and let it spark to the frame. while cranking. If you have spark then you have to start checking the fuel system.

I would first have the alternator checked by an auto electrician to ensure it is working correctly and is providing the correct volume of current. It may be faulty and when driving you will be drawing battery power instead of power from the alternator to run the electrics. This will drain the battery.

Alternators do not last forever and if you are still running the original unit it is likely to be on it's last legs and you will need a replacement unit.

95% of the time that means you need a starter, trying smacking it with a hammer while someone cranks the key

A definition: cranking is what happens when you turn the key, the starter engages and turns the engine. (term comes from when that was done with a crank that you stuck into the front of the engine to start it, before electric starters were invented) Turning over is the same as cranking. Starting is when the engine actually starts, even if for a brief moment.
Now...if the engine turns when you turn the key it is cranking. Was it doing that? If not, the problem is either in the battery, starter/starter relay or security system (sometimes) or the ignition switch. If it is turning and not starting, then the engine needs to be checked to make sure it has spark at the plugs, and fuel going into it. These problems are different ans some can be easily solved.

First run a code check, see what you find. Second, you can do individual component testing with a decent volt/ohmeter to pinpoint problem. (instructions for tests in any decent manual)
Just off-hand, most common failure with similar symptoms to yours is crank position sensor.

i had a similar problem with a gmc jimmy and changed ignition module several times before i ended up buying a new distributor and changing it out and have had no problem since 2 years ago

Same thing happened to me at about 140k. It was the fuel pump. Best way to test if your pump is failing and loosing pressure is to see if the car will start immediately after it has been running, or turn the key to the alt position for a few minutes before starting. If your engine fires faster after it has been running or the key has been in the alt position you will probably need a new fuel pump. Great news is with a very deep socket to get the tank straps off its a 30 minute job to replace the pump/filter assembly.