Unable to open mouth wide since orthodontia. Jaw muscle goes into spasm during long dental work. Need help asap?

Orthodontia. You need to call your orthodontist to evaluate reason. I can't diagnose the cause without an exam myself. A proper treatment can be determined once the cause is known.
Seek help. Were the braces just placed? It is a long procedure, wherein your mouth is opened wide for a long time. The word for your ailment is trismus, and is muscle spasm and inflammation. It will respond to time, moist heat, soft diet and advil (ibuprofen) (nsaids). Relax, it will go away.
Options. If your inability to open is strictly due to muscle spasm and not to a TMJ dysfunction, moist heat, and usually an over the counter anti-inflammatory such as Ibuprofen or Aspirin if you can take it, will help. You may also want to limit your diet to a soft diet with limited chewing until the spasm subsides. If that does not resolve your muscle spasm, a prescription muscle relaxant may help.
Go back. Go back to orthodontist for evaluation. Problem most likely muscular, but small chance it is jaw joint. Till you get back to orthodontist take analgesics (motrin/advil?Ibuprofen), eat a very soft diet, and apply warm compresses.
See your dentist. These symptoms point to an internal derangement of the TMJ and probable closed lock position of the joint. Inability to open wide is probably caused by a non reducing forward position of the TMJ disc restricting translation of the joint and opening. Concluding on the sensitive retro dismal tissues rather than the disc causes pain and muscle spasms.
Discuss with Ortho. Go back to your orthodontist to discuss tmj. They may refer you to another doctor to assist.