Reload the display adaptor driver.
Find out the Display Adaptor Chipset. It is shown under the advanced button in the Display Properties where you adjust the resolution. Download the latest driver available for Windows 98.
Generally you will then run the downloaded program and it either do the whole job or it will save the driver files where you tell it.
Then go to the Adaptor tab and Properties button. Driver, then Update Driver and point it to the location where you saved the output of the downloded file.