what to do if you lose your pickaxe in terraria

Answer 1

make a new one

if nothing else make a wood pickaxe and start mining again so that you can make a better one

Answer 2

If you lose your pickaxe by mistake or misplaced it. You should make another one.

If you cannot make another one. Just create a hardcore character and place your stuff inside your chest then die with it. And go back on your main character and take that pickaxe.