Started the machine up this morning, and now it's not heating. I tested the thermal fuse, and it's bad. It's an easy fix, but I just replaced this a year ago and I'm not sure why this happened - any idea what might have caused it?

A frequent cause of your fuse behavior is corrosion where the fuse clips to the wires. Corrosion makes them heat up, heat trips the fuse.
The wires and connectors need to be clean bright metal. Check that first.
A poor crimp is another factor. A crimping tool is a good thing to have.

Also check the pressure stat. If it sticks on, the boiler could over heat and possibly trip the fuse.

IF you find any crimps that are corroded, they should be removed and recrimped on pristine wire.

Thanks guys.

I ordered new fuses, put one in, and it heats up as expected. Connectors looked ok, but I got new ones and crimped, hopefully we're good now.

I think we can close this thread, this has been really helpful to me and I REALLY appreciate everyone's help.