HELP PLEASE!! SCORE INVALID

Please identify your computer's operating system and version, and MuseScore's version. You can get all three simultaneously by clicking on Help / About MuseScore from the main menu. Then click the copy button (two pieces of paper) next to the "Build" number. Paste that into your post.

Then describe in GREAT and EXCRUCIATING detail exactly what you do when you save a score, what you do when you try to open one, what you expect to happen at each step, and how the program responds instead of matching your expectations.

It's possible to include too much detail but you'd really have to work at it. It's very easy to include too little ... so include more :-)