I posted too soon. We had the system doing updates (including kernel updates). Nick pointed out that the kernel update might delete the driver so we held our breaths and waited. The system rebooted and gave me a login prompt. And then a pretty cursor and then nothing but a black screen.
Nick, luckily, was still here and was able to help. We did some research and found things that might have been it (problems with the Catalyst drivers breaking Unity desktop, for example). But it seemed odd that the login screen would appear but then X wouldn’t load for the desktop…
Just in case, I searched for new drivers, and found updated Catalyst drivers from AMD for Ubuntu. We installed them and lo! the Acer booted with a lovely login screen and desktop!
To get the drivers to be available from the terminal, I had to mount the USB device by figuring out which device to mount first using this command:
dmesg | grep -e sda -e sdb -e sdc
Then mounting the USB device proper:
mount /dev/sdb1 /mnt' (if sdb is the thing that the usb stick is)
Here are the commands I used to install the drivers:
chmod u+x amd-driver-installer-catalyst-13-10-beta-x86.x86_64.run