Buying the die to chase/clean up the threads-is EXPENSIVE-$50-$100
Not worth it
If you are a cheapskate-get a thin triangular file-harbor freight
and attempt to clean up the threads with it
Start somewhere-where the threads are "good" and work up towards to top
Yes you will be cutting out some metal-but so will a die
So take your time-it can work.
$5 file-triangular
Real cheapskates can do it with a table knife with fine sandpaper wrapped over the blade-file is better.
No point in paying a bike shop $50 to repair a $30 fork-unless it is a GREAT hard to replace fork

TAKE YOUR TIME
Pictures would help-sometimes threads are beyond redemption-but if it threaded off-you should be able to thread it back on

If the threads are really iffy-don't tighten the headset "too much"
Luck
Charlie