DIAGNOSIS The scanner flatbed cable or the scanner assembly itself has a loose connection or has failed. This is a known weak point in this all-in-one design.
FIX First, this is almost always a software/driver issue. Uninstall the HP Smart app and any other HP printer software from your computer or phone. Reboot your device. Download and install the full HP DeskJet 2855e driver package from the official HP support website, not just the app. This package includes the proper scanner drivers. During installation, connect via USB cable if possible to ensure detection.
IF THAT DIDN'T WORK If the scanner glass doesn't light up at all when you power on the printer, there's a hardware fault. Perform a full reset: turn off, unplug, wait 60 seconds, plug back in. If the light still doesn't come on, the internal cable connecting the scanner to the mainboard is likely disconnected.
HEADS UP The scanner often won't initialize if there's a paper jam error or cartridge error active. Clear any other error messages on the printer first. Also, the scanner only works when connected via WiFi or USB—not over Bluetooth.