“Please tell me I don't have to start over from scratch...”
You're writing a resume for applying to jobs. You've been working on it for over an hour, but you forgot to save it. The computer program crashed, and now you hope that your work was automatically saved so you don't have to re-write it. You think this to yourself.
Please tell me I don't have to start over from scratch...
This is an expression that people say when they're worried that they're going to find out about something bad happening. They say "Please tell me" that the bad thing isn't happening.
For example, if there's a guy who you really don't like, but you think that your friend might be attracted to him, you can say:
Please tell me you're not attracted to him.
In the example at top, the speaker uses "please tell me ~" when talking to himself. That's normal too.
To "start over" means to start doing something again.
Doing something "from scratch" means that you do it from the very beginning, without having some parts of it done for you.
For example, baking cookies "from scratch" means combining flour, eggs, sugar, etc. to make the cookies (instead of buying pre-made cookies or frozen cookie dough):
Want a cookie? I made it from scratch!
"Starting over from scratch" means doing something again from the beginning. You can use this phrase if you did some work but now you have to do it over again because you lost it or because there was a problem with the first version.