If the ammo that got stuck in the chamber was something other than match-type ammo, it shouldn't be altogether surprising if from time-to-time one gets stuck in the chamber.
The Anschutz 64 single shot will have a chamber that has smaller dimensions than the typical sporter repeater rifle. For example, the length of the chamber will be shorter.
Non "match-type" ammo is not always made to exacting specifications, so their dimensions can vary from round-to-round more than match type ammo. In addition, the driving band of some ammos is wider and engages the rifling more fully than others. This can cause it to be hard to extract (and even close the bolt on) in a rifle like the Anschutz 64 shown.
If the rifle has been neglected or poorly cleaned, it should be cleaned using proper methods to avoid risk of damage. Use a bore guide (to avoid damaging the chamber or bore) and a one-piece rod. Push several solvent-soaked patches through on a jag from the breech end with the bore guide in place. Let the solvent-coated bore sit for a while. Then use a .22LR brass brush with solvent and push through in the direction of the muzzle a number of times.
This should remove most if not all of whatever may be in the bore but doesn't belong. The only other thing you may need to worry about is a carbon ring that may have developed just in front of the chamber. You can't see it by eye alone. If you suspect there is one, or if you wish to make sure the area is clean, soak a bore mop with a carbon remover product. Using a bore guide and rod, push the bore mop into the chamber and a little beyond. Let it sit there for a while. Then push the brass brush through a number of times. There are other ways to let carbon remover "soak" in the chamber/leade area.
Keep in mind that the name Anschutz (or any other name) isn't a guarantee that it will shoot very well with any .22LR ammo it's given.