If it is a very recent upgrade then try the interactive (default) mode of
aptitude run with
sudo; that has a TUI (text a.k.a. console user interface, done using
ncurses if anyone is interested) package manager that can display all the versions currently available/installed of each package.
If you can identify which package is the problem and you can obtain an older version (as a
.deb) file you can plonk it as root into
/var/cache/apt/archives/ and that application will allow you to change the version installed manually.
This is assuming other dependencies aren't an issue (the bottom area of the application turns from normal blue to red if the current combination of things is problematic) and other lines of listing. The UI may take a bit of time to get used to but for starters I will tell you that
<cntl>-t activates the menu bar and
q for quit will escape out of things; the current "line" is highlighted in reverse and any line begging with
--- can be selected with
<enter> to be expanded (when it changes to
Hope this helps you to put your system back together..., here is a couple of screen-shots of aptitude in use showing a package that I HAVEN'T installed "solaar-gnome" (non of the four versions listed on the bottom of the second picture is installed - there is just a leading
p indicating a package version is available) on the machine concerned (my Debian PC) as it would need a gnome package "gnome-shell" which I haven't got installed (the red line)!