It doesn’t matter how it happened. Maybe it was during the self-clean cycle or from an errant baseball in the house. Regardless, the glass on your oven has been broken. It has likely not shattered but manifested a crack in various degrees of severity. While you certainly intend to replace the glass, maybe that is not so high up on your things-to-do list. Can you still safely use an oven with cracked glass?
The answer is yes. The good news is that ovens often come with multiple layers of glass. In older models, you may only find two panes of glass. In newer oven models, they can have as many as four layers of glass for energy efficiency. In most cases, only one layer will break, which means you can still usually use the oven safely.
What you will want to do is heat your oven up to a toasty, but safe 350 degrees. This is the standard bake temperature for most things. You will want to feel the exterior glass and oven door during this process. If the oven door or glass is dangerously hot, as in it hurts to touch it for too long, you will want to make the replacement of the glass sooner than later. That heat means that the heat from the interior is making its way to the exterior of your oven, and this is the problem that you were likely worrying about when deciding if it was safe to use your oven.
If you touch the oven door or the glass and feel only warmth, not painful burning, you can continue to put oven glass replacement off for a while, but you are literally playing with fire. However, you will want to realize that this issue can get worse and your oven’s exterior can become unexpectedly hot. As such, you will want to monitor it.