My Canon does that when I try to turn it on AFTER I sent the print job to it...It then seems to spit out endless pages of garbage characters.
The easiest solution that should work for you:
1) Turn off the HP OfficeJet
2) Bring down the PC
3) If the printer is hooked up to an external Jetdirect or other print server, shut that off too.
4) Wait several minutes as the buffer may be able to keep the job in memory for a short period of time.
5) Finally, bring the devices back online in this order: Print server, Printer, PC.
Use the Windows "Test a Printer Connection" to make sure that it can now print normally again.
If it still does not print right, I would recommend checking with HP at their website. You may have to set up a login name for their support service (free) before you can access their FAQ or support documents.