Your key worked fine yesterday! If it isn’t turning today it is probably because the steering lock is engaged. Try turning the steering wheel hard to one side and the other while trying to turn the key.
Try sticking your key in the ignition and tap it on the end with a hard object like a screwdriver. Also try a small squirt of lubricant like triflow or 3-in-1 oil but not wd40. Squirt it right down the keyhole and then move your key in and out several times to spread the lubricant around before trying to turn your key. If you are in the middle of nowhere try running your key along your dipstick to transfer some oil onto it. A little bit of oil is all you need.
If you have a Honda or Toyota you may be having problems with split wafers. These are to make it hard to pick the lock. They can wear down and bind inside the lock. Eventually they have to be removed.
Whatever you do, don’t force the key to turn. The wafers in your lock were designed to be stronger than your key so that people can’t just turn your ignition with any random key. It costs much more to remove an ignition with broken wafers and a broken key in it than a working one!