This is totally weird and only happens occasionally, but is a total pain. I go to start the vehicle, there is a loud clunk, and then the vehicle has no electrical power, and we can't get the key back out of the ignition. Here is where it gets really weirs. If I take my 12 volt tester and test the battery, as soon as I touch both battery terminals, the power in the vehicle comes back on and we can now remove the key. but sometimes we have to go through this several times before the vehicle will start and operate normally and then it will be weeks before it happens again. I might mention that bout three months ago the ignition switch was replaced. But this only started a month ago and it doesn't make sense to me, no idea how to troubleshoot it. I'd appreciate any ideas.

 

This is a bit of a stab in the dark, but I had a similarly intermittent issue, but for me it was not being able to start although it would crank.

It turned out to be the fuel pump relay that had bad contacts due to repeated use over almost 25 years and due to the relatively high current that goes through it. These relays are in the under-hood fuse box. A quick thing to try is to swap the ones that get used a lot with larger currents, e.g., the one for the fuel pump, with ones that don't get used much and draw smaller currents, e.g., the relay for the rear defroster.

Even if the odds this fixing it are low, it is super easy to check! You don't even have to get dirty to try this. LOL

I later got new replacements for all of them just to be sure.

 

Building on Christine's advice, another simple thing to check is your battery terminal ends. The battery cable ends can get corroded over time and build up resistance. Putting your tester on the terminal might disturb the alignment enough to make enough connection for the vehicle to work. Then over time the vibrations and use builds up the resistance until it won't start again. While the vehicle is running the alternator is powering the vehicle so you wouldn't even notice if the battery wasn't making connection.

Taking the terminals off and cleaning them carefully with a wire brush and maybe a baking soda & water solution might help with ensuring a solid connection.

 

I have a very similar scenario with my 02. I have this happen at least once a month. It turns out the negative battery terminal slightly shift, causing a loose connection. I just pop the hood and push the cable clockwise a bit and bam all power comes back. I have replaced battery and have zero corrosion at the terminals. I am hesitant to over torque the bolt in fear of snapping the head off. Does not happen to positive, only negative. Cant tell you why. Hope this helps.