Brobably the best place to look for what happened when you tried to start X is at the logfile:
Hopefully that gives some new information.
It might depend on what you upgraded, so try to remember which packages you installed - video drivers, perhaps, or a kernel upgrade? Are you using some proprietary drivers (like ATI's fglrx or nVidia's equivalent drivers)? If you are, you might have to reinstall them, then reconfigure your X.
You may also want to review that the configuration file of X is ok:
If you need to make changes, log in as root (in the console), then use vim, nano, pico, joe or any of your favourite editors to edit the file. You could basically use one of the semi-automatic X-configuration "programs" (scripts usually), but they mainly just overwrite the whole configuration and ask you loads of questions, and therefore it's often easier to just directly edit the config file if you know what to change.
Well, see the logfile if it helped to locate the exact problem.
You could also try to start X with xterm only, so you saw if it was the desktop environment (probably not) that causes the trouble, or X itself (it's configuration usually):
echo xterm > ~/.xinitrc
The above basically renames your .xinitrc file (which is in your home directory) to .xinitrc.bak (so we don't lose any data that might be there), then creates a new .xinitrc file that has only "xterm" in it (so when X starts, it just runs xterm and when it ends, closes X), then startx tries to start X (.xinitrc is read, and if X starts, xterm is launched). If it works, just type 'exit' at the xterm prompt and you're back at command line - and know that X is not the problem, but something in your desktop environment (I doubt this). If it doesn't start, then you'll just run the last command putting the original (renamed) .xinitrc back to it's place and think of something else.