The Officejet J5780 has a error message of out of paper@

First of all make sure there is no paper pieces stuck inside the printer. Now ..!

Unplug the power cable ofthe printer.
Unplug the USB cable from the Back of the printer and make sure it is stand alone printer.

Now,
Take out the duplex-er ( A box at the back of the printer which has two buttons at the ends of it which you can press to detach it from the printer :- It is used for duplex printing ) from the back of the printer.
Once you take it out you can see the ROLLERS and the ROD which takes the printer page and rolls back to give you the out put.

Take a lint free cloth and clean the rod and rollers. Try to move the rollers in clock wise or anti clock wise direction. If it moves fine Other wise hardware issue.

Now attach the Duplex-er back to the printer.
Now Reset the printer:-

To reset it.
Connect the printer power cord to the Wall Outlet or Wall Jack.
Make sure it is not connected to the Serge Protector or power strip.

Partial reset :-
Now on the printer itself. --> Press and hold the
# and 3 button at the same time. Now take out the Power cable from the back of the printer and wait for 20 sec. Now put the power cable back in the printer. Now release the two buttons. ( Note :- remember you need to hold down the # and 3 button the hold time )
(On the printer screen it should say Part nvm reset.)

Now try to turn on the printer and check if it shows the out of paper option. If it works Perfect. If not then do this :-

Semi-Full Reset:-
Open the front door and take out all the cartridges from the printer. Now close the door.
Now on the printer itself. --> Press and hold the
# and 6 button at the same time. Now take out the Power cable from the back of the printer and wait for 20 sec. Now put the power cable back in the printer. Now release the two buttons. ( Note :- remember you need to hold down the # and 6 button the hold time )
( On the printer screen it should say Semi-Full rest )
The printer might shut down automatically or may be it won't. At this point Put the cartridges back into the printer and then turn on the printer if it was shut down.or If the printer is on turn it off and then switch it back on.

The problem should be resolved up till now.
If still the problem Exist Call HP as the product has gone bad.

HP :- 1-800-474-6836 ( Tech Support )