Time Machine Backups Disappeared Off the Hard Drive

Last week, I went into my Time Machine to find a file I knew existed less than one year ago. As usual, everything was loading very slooooowly, but I did manage to find what I was looking for. My backup files went back about 3 years.

I also noticed that I had set the frequency for backups to be "every hour" and since I no longer need that kind of frequency, I changed it to "each day."

Now, I'm horrified to see that 3 years' worth of backups seem to have disappeared and what's left is only for yesterday and today.

How could this have happened?