Pro stopped and I don’t know why

From the app’s Help panel

Pro stopped and I don't know why

Settings → License → the line under the key box says exactly why: the key reached its end date, was refunded or cleared on another computer, a code was used more times than it allowed, or Tentaku couldn't check your key online for three weeks. Connect once, or paste your key or a fresh code, and Pro resumes. Your notes are never touched.