Open Hot on GFCI outlet
Sounds like the GFCI receptacle got wet (or perhaps a downstream receptacle if there is one), and something is shorted out, tripping the GFCI. 

If you have any Christmas lights still plugged into the outlet, then something may have shorted out in the lights, making the GFCI trip. Try unplugging everything and see if the GFCI will reset. If it does, then dry out and inspect the lights carefully to look for problems -- don't plug them into a non-GFCI outlet since there is clearly an electrical problem and use care around them until the problem is found.

If it's not a problem with anything plugged in, then new GFCI's are cheap enough that I'd just try replacing the GFCI (fixing any water intrusion problems first, if there's water in the box, then figure out why and stop it from getting in there). If the bad GFCI eventually dries out and starts working again, I personally wouldn't trust it again.

This is likely not a serious fire hazard, but could be an electrical hazard (since something is apparently shorted out), so to be safe you could turn off the breaker that powers that outlet.

If the outlet got wet because the lights were plugged into the outlet while the outlet cover was open, you can purchase an outdoor outlet box that allows a cord to be plugged in even when the outlet cover is closed:



(this is just a representative sample, there are other styles and manufacturers of similar products)