You borrowed from your 401(k). It felt fine. You were paying yourself back, the interest went into your own account, and the payments came straight out of payroll so you barely noticed them.
Then the job ended.
And somewhere in the exit paperwork there is a sentence about your loan becoming due, and suddenly a very reasonable decision from eighteen months ago is about to generate a tax bill.
Here is the whole situation, start to finish. It is more fixable than most people realize, and the fix has a deadline almost nobody knows about.