The muscles on one side of my jaw are very tight today. I can't open my mouth? What could it be?

Trismus. What you are describing is caused trismus, which simply means limited opening of the mouth, which can be due to many things, including tmd (temporomandibular disorders). It could be muscle spasm, perhaps due to bruxism or clenching, or it could be a displaced disc inside the jaw joint, or tmj. This is commonly called a closed lock, and if it persists, you should see a specialist.