Ethereum's POS has a concept called finality which means that under certain conditions the blockchain gets checkpoints that can't ever be reverted. Somewhat simplified but the the blocks are finalized if 2/3rd of the validators validated the last 64 blocks. So, normally after about 13 minutes you can be sure if the transactions are set in stone or not.

If the cut off validators are reunited after not too long, they won't have been able to finalize any blocks and then they will just throw away the part of the chain they have been building since they were separated from the majority. As far as I understand it that's quite similar to a reorg on a POW blockchain.

The other case, if they've been separated for a very long time, the situation is a bit different. If you don't follow your duties as a validator, you get penalized. A little of your staked ETH is burned. Note, that's not the same as slashing (which is much harsher) and if your staked ETH balance falls below 16 ETH, you're kicked off the list of validators. This is so that active validators will be able to reach again a 2/3rd majority after a while and be able to finalize blocks. If this happens to our cut off validators, even if they get access to the other validators data again, they can't reorg the blockchain as their blocks have been finalized.

In this case, the admins need to manually reset the data so the validators can rejoin the correct chain. This is a known issue which POW networks don't have. There you can always rely on the chain that has the most work done. But the work is cheap and fast in POS networks so you sometimes need to get external info and verify it by hand.