Why did my letter c not work 15 minutes ago?

I have a laptop corei5 Aspire S3 windows 8.1, and my letter c stopped working for no apparent reason. I did a virus scan and it didn’t find any viruses. qwertyuiopasdfghjklzxcvbnm works for now. I need the letter c for my password, So I could use help in case it happens again.