The prdownloads.sourceforge.net URL often does not work.
My solution was to edit the file in at var\lib\mingw-get\data\profile.xml and change the <repository uri="...> line to:
<repository uri="http://sourceforge.net/projects/mingw/files/Installer/mingw-get/catalogue/%F.xml.lzma/download">
According to this issue thread this may be an issue of a SourceForge. I've tried to install mingw using mingw-get today and the problem still exists.
I stopped using http://mingw.org/ ages ago as it was getting behind on newer Windows API additions and newer gcc versions.
Instead I really recommend MinGW-w64, which supports both Windows 32-bit and 64-bit.
If you want to keep your old MinGW on your system and would like to give MinGW-w64 a try without interfering with your installed version, you can download a standalone build from http://winlibs.com/ that you just need to download and extract.
If you use MSYS shell you should also switch to the newer MSYS2, which is a lot faster.
I guess they've stopped paying for this domain.
The good news is that MinGW is still available on MinGW - Minimalist GNU for Windows download | SourceForge.net. Note that the maintainer announced on the homepage:
This project is in the process of moving to osdn.net/projects/mingw, you can continue to follow us there.
So you may need to go to MinGW - Minimalist GNU for Windows Project Top Page - OSDN.