Troubleshoot
The monitor is probably going defective as a result of age. Take the following steps in order to be sure nothing else is to blame:
- Reset the Monitor (using the buttons on the monitor itself, NOT the operating system).
- Connect different cables, both power and video (DVI/HDMI).
- Try a different monitor (if you don't own an additional monitor either borrow one from someone else, or bring your computer to them).
If none of these steps solve the problem it is very likely that the monitor is at fault. Use the warranty if still applicable.
Details
It could be the power supply but more likely the capacitors. Another symptom I've encountered is the monitor flashing slowly when just turned on (cold state) and it gradually increasing in frequency (strobe effect), then after a while it stops and is usable again.
Also, modern day monitors report all supported modes to the connected device (via digital output / input). The fact that you can still select 32-bit within the control panel convinces me even more that it is defective.
It is very unlikely this happened because of installing an older game on Windows 10. Switching modes upon launch might have been the tipping point, but age is probably the real issue.