I decided to get back into The Sims 2 for a bit lately for the sake of nostalgia and loaded an old save in a custom neighborhood.

Everything fine and dandy, until the game started crashing pretty often because I'm now running Windows 11 and Direct X is stingy so I got an RPC mod that fixed the crashing.
The game worked fine afterwards, played two families, raised kids and last time I played them I saved as usual and went my merry way.

Today I wanted to play again, so I loaded said neighborhood.
Lo and behold! Everyone was gone! All the houses were empty (for sale) and when I tried to load one of the houses, the game crashed!
I was actually quite pissed at that. I really like those families and the progress I had with them.

I reloaded the game again and went into a different neighborhood that had pretty much the same families from the other neighborhood and built something there.
Then when I was done building, I decided to check on the other neighborhood and to my surprize, all the households were back... hoooray?

Here comes the weird part:
I excitedly clicked on one of the families and loaded them. First thing I noticed was that their aspiration bar was rock-bottom, then I noticed that their icons were in a random order.
Then, I clicked on a child's icon and said child wanted to get a job... just to realize right away that everyone in the house was marked as an Adult, despite looking like kids/teenagers!
Their personalities were completely gone, all the bars were empty. Family tree, also vanished.
Also, they all had male voices!

So I went to check up on my other household, which had a dog as a pet and that exact same thing happened to them, plus that female pet dog also counted as a male adult sim who wanted to get a job!

All the other households that I wasn't actively playing that I checked up on, suffered the exact same fate.

Next thing I did was move them out and in again, thinking that would save them, but no. They stayed broken.

Then I moved in a bin family into a lot and they acted normal.

I'm not exactly sure what happened to this particular save, the sims were gone and came back irreversibly bork'd, but I'm worried that the same thing will happen again.
This had never happened to me before and I've played this game for a very long time!

Oh no, sounds like you have something interfering with your game reading and writing to your files.
Check-
Do you have cloud storage (e.g One Drive) synchronising your Documents/ EA Games folder?
Do you have low disc space?
Are you on a Mac?
Do you have an antivirus set to on-access scan your Documents/ EA Games folder?

Like simsample said, to me it also sounds like interference. OneDrive is known to work particularly badly with Sims 2 so most of us have it either removed or completely disabled. Some antiviruses will also mess the game around by removing or quarantining files because it doesn't recognize them as being a legitimate program due to its age. You have to set them to be white listed or ignored. Any old game can be affected by these types of programs.

Do check how much room you have on the drive as the game needs space to save.

I run on a 16GB ram computer and have 635 GB of free space, so no, I don't think space is an issue.
As I said, I am on Windows 11.

I found OneDrive and removed it though as it could have been the culprit.

Sorry, I completely missed where you said Win 11!
The neighbourhood you were playing may be corrupted though (as in, have data missing) so you might have to revert to a backup or start over again. Be sure to make a safe backup once you set up your hood again!

Empty bars and all sims being adult males sounds like one of your crashes trashed some data. If you have a recent backup, it's probably your best bet to restore that.

For whatever reason, the Sim Descriptions (SDSC) from the neighbourhood package disappeared. When loading up the 'hood again, the game notices unassociated character files and generates new SDSCs for them, which are all adult male.

Unfortunately, this means your hood's trashed. After you figure out what the cause is (OneDrive is known to be a culprit, and aggressive antiviruses), restore from backup if possible, or extract their appearances in SimPE to re-create them in a new hood.

More information from another thread, though the part about the Default family doesn't apply in this case: