Pedals spin, but wheel doesn't

[grousemaster]

On a Trek 700 hybrid, about 10 years old, my back wheel suddenly did not respond to the rotation of the pedals. Heard a lot of clattering from the cassette area, too.

Suspect the grooves in the freewheel hub are worn out, or the ratchets that engage them are broken or worn.

I don't think replacing the hub body is any big deal but am not sure about the ratchets (or pawls?). Should you/can you replace both? And should the cassette be replaced at the same time, too?

Would anybody diagnose this problem differently? There was no problem shifting and the chain was fully engaged both front and rear...

[cyclerUK]

It sounds like the freewheel / freehub pawls are either sticking or worn.
You could try getting some thin release oil or WD40 in, to see if they free off.
Otherwise it's a replacement part required.