My Iron Golem got himself stuck inside of a mine cart
Solution 1:
Can you place rails under it or push the golem-cart onto nearby rails? Build a short little track that ends in an Activator Rail which will eject mobs from the minecart as it drives over (including your iron golem)
If not, you can still build your track underneath, and then dig out the block the minecart is on to make it fall onto the Activator Rail.