Bruvra

I am a new user. I am able to generate a treble clef with tablature for guitar. I am doing fine, placing notes in the lines and below the lower e in treble clef, but all of a sudden I cannot place notes, for example, a C note below the e line in treble clef. What have I done to lock this up for me? How do I reset it so I can write low notes? Hopefully, you can understand my problem and have a solution. Thanks.