DIAGNOSIS The printer's internal memory has hung on a corrupted print job, or your computer's spooler is stuck. The 'processing' state means it's received data but can't interpret or execute it.
FIX The fastest fix is to restart the printer's network connection. On the printer, press and hold the Wireless button for about 3 seconds until the light blinks. This disconnects it from the network and kills any pending wireless jobs. Wait 10 seconds, then press the Wireless button again to reconnect. Alternatively, just power the printer off and on. The stuck job should clear.
IF THAT DIDN'T WORK You have to clear the queue from the source. On a Windows PC, go to Settings > Devices > Printers & scanners, select your HP 2855e, click 'Open queue', and cancel all documents. On a Mac, go to System Settings > Printers & Scanners, right-click the printer, and 'Reset Printing System'.
HEADS UP Printing very large PDFs or high-res photos can cause this, as the printer's memory buffers overflow. For big files, try printing in smaller chunks. Also, switching from 'WiFi Direct' to your main home network in the HP Smart app can prevent these hangs.