security – What prevents somebody from “re-mining” an already existing Bitcoin?

This may reveal that I know nothing about math or cryptography.

Let’s say that a given Bitcoin was “lost” in that it was once mined and assigned/given to a person who had it in their wallet.dat, but then they either lost the password to this file, or deleted it, or otherwise lost access to it forever (or without any realistic hope of ever retrieving it).

Now, that Bitcoin has been mined out of the 21,000,000 total ones ever, but it’s “lost”.

But if a computer once guessed the right private key for that coin, why can’t another computer do it again?

Basically, isn’t it possible that all the Bitcoins that are currently mined and in possession of somebody, could be “re-mined”?

Is it just “extremely unlikely”? And you cannot “target” a specific existing private key/receive address/Bitcoin?

I’m always fearing that one day, somebody “re-mines” the Bitcoins that I have, thus seizing access to them, either by sheer accident or by somehow deducing the ones I own and specifically try to “re-mine” those in order to make me poor?