Acer AL1916W suddenly not showing correct resolution.

Today I restarted my mac mini running Yosemite.

Now my screen resolution is messed up.

I have a choice of 1280x1024 or 800x600, but this monitor, an Acer AL1916W requires 1440x900.

It has worked fine for over a year, but just with this restart it does not.

I thought maybe the driver suddenly disappeared, but I cannot find a mac driver for it.

I tried a second restart, which didn't change things. Next I will do a full reboot, but after that I am stumped.