Why did my Steam download restart back to the beginning?

Solution 1:

Steam does not track download progress if the computer shuts down. When you turn your computer back on, it will say that it is starting over from the beginning, but I believe it will check to see if it already has each file before it downloads it again. If it finds the file, it will move on to the next file instead of re-downloading it, so it should move through the download process much faster the second time as it is just finding all the files again until it gets back to where the download was previously.

In my opinion, it is best to leave your computer on until all downloads are completed, but it does not start over completely if you do pause the download or shut off your computer. It just takes a bit of time to figure out where it was before continuing the download and installation.