You should get the miles from the airline you actually flew if you gave them your FF number before boarding. It should happen automatically. If you didn't, or if it doesn't, you can follow their standard "missing mileage credit" process.

Getting mileage from the airline you were originally booked on is a controversial topic that has been debated (perhaps "argued" would be a better description) ad nauseum in other FT threads. Some, like the previous poster, feel it is only fair compensation for the inconvenience you were put to. Others feel it is taking advantage of the fact that airline computers don't generally talk to each other, since it is against the rules of most programs. I'm not taking a position here, I really don't want to restart that topic, just noting the issue.

If you want to get credit from the original airline, it usually takes a special request since standard "missing mileage credit" processes are not designed for when you didn't take the flight. If you use the words involuntary reroute and request original routing credit (using the exact words in bold type) when you contact them, you will usually get it. You may be told it's a one-time exception - a precaution on their part to prevent it from becoming a legally binding precedent for the future - but it's not all that unusual.

Other helpful things to add when requesting ORC are that the reroute was due to a mechanical failure, crew unavailability, or something else under the airline's control if that was the case, and that you didn't request credit from the other airline as well, if that was the case. These shouldn't be necessary but may make a difference in borderline cases.

It's a bit too late for this now, but for the future, it can also help if the agent who reroutes you adds a notation "give original routing credit" to your PNR.