Flight Attendant reported the gate agent advised them the exterior cabin door handle was in the open position after landing.

Narrative

Door closure procedures were all correct; the gate agent closed the door very well with no struggle at all. All calls were completed with an additional verbal confirmation that all door handles were pushed down; including mine. Just after 10;000; I heard a distinct 'pop' sound coming from my door and a change in the frequency sound of the white noise. I checked my handle; it was down and had not moved. There was no whistle sound; no indication of pressure change; no indication of any physical change in the door. When we arrived at the gate in ZZZ1 [airport]; I noticed the gate agent taking pictures of the door before moving the jet bridge into position. When the door was opened; she showed me that the outside door handle was completely in the vertical open position. I immediately went to the captain and told him what I heard in flight; what I saw and he confirmed to me with no concern (at that point) that had not received any notifications of any issues at all. Then I told him about the outside handle and the picture the gate agent took. He left the plane to see the picture and when he came back and shared the photo with the first officer; they both very clearly had looks of serious concern on their faces. They immediately began making calls and reporting the issue to maintenance. I asked the FO if this was indeed a serious issue and he said yes; admitting that this plane was to go right to the hangar for inspection. At that point my crew and I left the plane; and I warned the new crew coming on that they should not fly on that plane. I'm sure with no doubt that this was some kind of mechanical failure of the door. I know the door was closed perfectly; the handle was down and there was no struggle at all with the door closure. In my opinion; something broke or failed with the outside handle after we reached 10;000 feet.