External light was not earthed properly
Fixtures are grounded to protect one from getting shocked if there is a short or fault in the fixture. The ground wasn't connected properly, according to you, so you got shocked because of some problem. Now that the ground is properly connected, you shouldn't get shocked but the problem still exists and must be fixed. Examine the fixture for frayed wires, cracked insulation. Be careful if you're up on a ladder and make sure to turn off the power. If you don't feel comfortable examining it, just replace it.