2005 X5 3.0i sport 117k miles

Going to work this morning it was blowing icy cold air to a point I got cold. Coming home I hear sorta hissing sound from the vent then it started blowing warm air as it's 90 degrees outside. I've never had an A/C issue during the 10+ yrs I owned this car.

What's the first thing I should check? I opened the hood but don't see anything unusual.

Sorry, I should search for old posts but I'm posting using my phone and the internet's very spotty as I'm on the road currently.