Can not enter notes

MuseScore 2 has stopped allowing me to enter ANY notes. I've tried everything. I write rhythmic assessments on a single line and have written 87 already, but it just stopped allowing me to enter notes of any duration. I've done all the fixit steps. The blue box appears, a blue note appears, but no matter how many times I click the staff, nothing happens. Keyboard entry, the same.
HELP !