Internal Error & All blocks deleted

This problem started happening for no reason, as I did not change anything on the project and it started to happen, probably after some update. My images are light My project is smaller than 5mb

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after getting the first message you should click "Restore the previous version", else your screen will be empty later

see the following general tips and in your case especially #6

1. Use different screens wisely

Before starting to create another screen, first you should think about is it really necessary? See also Building apps with many screens and SteveJG's post about advantages/disadvantages, because in only one screen you also can use vertical arrangements to simulate different screens, just set the arrangements to visible = true/false as needed... See also Martyn_HK's example about how to use Tabs in App Inventor and another example from Cyd. 

The recommended method of switching screens in App Inventor

manager.aia

2. App Inventor works best if you use images whose size matches the size you want them to appear on your screen. If you import larger images into your app, your app may run out of system memory. Using Images with App Inventor

3. Avoid redundancy

Probably it helps to read chapter 19 - 21 in Dave's book http://www.appinventor.org/book2 to get an idea how to do DRY programming with App Inventor - Don't repeat yourself 

4. See SteveJG's monster list of tips and tricks

5. How to overcome the App Inventor project limit of 10 MB

6. Backup your project frequently

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It's not work

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import your latest backup and restart from there

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