Besides the medication to clear it up, you will need to scrape out the lesions that may be blocking her tongue and crowding her airway and esophagus. This is not life threatening in itself, but a chicken can suffocate or choke if the lesions become a blockage.
You need to be brave about doing it, and it may cause bleeding, but it's necessary if you don't want your chicken to die of suffocation or starvation because she can't swallow food down.
This is usually transmitted to chickens by pigeons. If you have wild pigeons hanging around, you need to keep all water bowls and other water sources from being contaminated by the pigeons. If you have other chickens, you may need to treat them all since this is contagious.
If it is canker, an anti-protozoan drug Fish Zole (metronidazole or Flagyl) can treat it. Dosage is 250mg daily for 5-7 days. Ronidazole is another possible treament available online for pigeons. Some also treat water with acidified copper sulfate 1/4 tsp per gallon for 3 days each month for prevention. Clean and disinfect all waterers and feeders. Here is some reading: http://www.poultrydvm.com/condition/canker https://www.chickenvet.co.uk/oral-canker