Assume the chain as part of the chainwhip snapped.
Can I enquire: why do you feel the need to replace the cassette? If the cassette is well and truly on (removal a separate issue - advice upthread and I must confess never having experienced a lockring on that tight (meant to be 40Nm)) why can't you ride your bike?
You've got it in one...
It was a combination of poor planning and procrastination: I knew the chain and cassette were getting worn, but planned to leave them working together until winter when I'd put a new chain on and put on the winter wheel with an equally new cassette. Of course, I'd said this about last winter, and now I come to thinkof it, this summer, but hey, the problem wasn't urgent so I kept forgetting.
I then compounded this by forgetting that the cassette which was worn with the chain was on the winter wheel and just swapped them over, which of course meant that first the smallest and then the second smallest gears began to slip. Of course I ignored this up until I had trouble accelerating from a traffic light a couple of weeks ago and since then I've been collecting parts and tools,one of which I promptly broke on Saturday.
The problem was I'd already fitted the chain, so it just rattled over the cassette without actually moving anything...
Today I went to the LBS with some trepidation as they aren't always that helpful, but the guy took the wheel and applied his shop-standard chain whip, then called a colleague over to help, and finally went and found a length of what looked like scaffolding at which point the lock ring gave up. He didn't want paying either but I put some money in his tips jar...
Now we have the new cassette on, new chain on, and I've got a commuter bike again...
Having compared the old cassette with the new one I'm not surprised there was slipping, in fact I'm more surprised that it held at all: the teeth on the smallest two rings are half width...
Now, will I take the advice given here and get to grips with maintenance before it all goes to pot again?