Why can I not delete blocks and things in Minecraft anymore?

Minecraft Bug:

I wanted to build something in Minecraft, saved it and quit to see something about Minecraft, and when I came back, I kept building and built something wrong. I tried to delete it, but it didn't let me. I can still place things, but cannot delete them. Why can I not delete things in Minecraft anymore? Could you please fix that or help me out? Maybe I did something wrong but I don't know what... Thank you.

 

                                                                                                           Anne (MinTYPlayzLOLLY)