HP Scanner only scans black now?

I just re-attached my scanner after more than a year of it sitting by the computer as a shelf. It’s an HP Scanjet 2100c. When I first hooked it up about a week ago, it worked beautifully- the color was even closer to true than my digital camera, which made me very happy. Unfortunately, the last few times I’ve tried to scan something, it acts like it’s scanning, and the scanner bar moves, but what appears on the screen is blank white, which after the scan changes to blank black. It used to do a scan, then adjust the color after the scan, so the color change is what it’s set up for- but why white and black and no picture? Any thoughts on what could be wrong?