In normal operation Ubuntu does not utilize a lot of disk space for itself, even with a bunch of packages installed. On a filled 50 GB partition, there are surely some warez you can remove or temporarily move somewhere else.
However there is another option:
tune2fs -m 0.5 /path_to/device
From the tune2fs manpage:
Set the percentage of the filesystem which may only be allocated
by privileged processes. Reserving some number of filesystem blocks
for use by privileged processes is done to avoid filesystem fragmentation,
and to allow system daemons, such as sys‐logd(8), to continue to function
correctly after non-privileged processes are prevented from writing to the
filesystem. Normally, the default percentage of reserved blocks is 5%.
So using this command you will lower the the reserved blocks in your example from 2,5 GB to 250 MB, which should suffice to boot Ubuntu again.
Obviously you need to run the command from the Live CD. Put the setting back in place after you fixed your issue(!) with:
tune2fs -m 5 /path_to/device
As I said in normal operation Ubuntu does not fill a 50 GB file system to the brim. If that 5 GB and above game was the only large thing that you can remember to put on this file system you might want to investigate here. You can use the graphical disk usage tool that comes with Ubuntu or use a command like this:
find -size +1G /path | more
Files over 1 GB in size are very rare in the Ubuntu system. Also take a look into /var/log or the log viewer. Don't just remove the files, try to find out if they have to be that big or what made them grow. If it is a logfile take a look at it. I had one with 30 GB in size due to my KVM switch acting up. Replugging it fixed the issue.