Can a acl reconstruction surgery come undone after 2 years or can the screw come undone?

Yes it can: An acl reconstruction can fail. It is not any stronger then your acl ligament was prior to the initial injury. So if you plant and cut on a planted foot or other traumatic incident it can re-rupture. If an allograft is used there is a slightly higher re injury rate. Further sometimes but rarely the screw that is used to fix the graft an come loose. You should see your surgeon for follow-up.