I had finished a level but had tons upon tons of unclaimed powers, so I used one of my multi-level powers only to find I basically just lost it because it won't let me multi-level with the completed level.

Oof, yeah, that's something only Fizzer can fix. I have an extra Multi Level I'd give you if I could. Condolences on the loss

So does this mean I don't get my multi-level back?

No promises, but there is no automated process for this. Mail Fizzer directly and ask politely, that might work. And might push him to improve the situation altogether. ;) We could all benefit from this in the end. For example I could imagine that the level you start with ML is flagged as such and doesn't count towards the levels you can start. This way the effect of the ML power would only end when you finish the one level that you used ML on. If you finished your "regular" level in between you were able to start another in parallel. I'd argue that this would feel more natural, reduce frustration, and I imagine wouldn't cause any real downsides for Fizzer (losing out on income, etc).

Alright, I messaged him. It is a legit glitch that needs to be fixed if you can't multi-level after completing a level but can still use the power.

If your intent is to leave the level around for completing digs, fulfilling clan reqs, etc, you're best to leave a few territories unclaimed (at least 1) so it's never finished, then it won't disappear from your ongoing level list

@krinid, while this is a valid suggestion, I'm pretty sure that this wasn't the problem here. But I could be wrong.

I am not sure whether it will help you, if we joint Battle (using Battle icon), after its completion we will return on the 'Level List' where we can select one level we want to go from our currently active levels (there are multiple Resume to Level buttons). I have never start a new level this way yet, just switching between levels (from completed one to on-going one). But I think we will able to start a new level too. But the Battle need to be started, if it is cancelled then we will return to the same level we joint the Battle from.