Your problem is more than likely that the sensor lever is not properly positioned. You may also have dislodged the shaft from the drive gears (gears on right out side of carriage). Both are easy to do.
Here's a couple pictures of the exact positioning of the paper sensor positioning. It should freely flip up and fall back down with gravity. Remove the tray and check the flag that hangs down.
Just in case you have dislodged the shaft from the geartrain, here's a free copy of the HP Service manual for the printer that will walk you through disassembling the right side for re-mounting the shaft and holder clip for the shaft.
Hope that helps you. :)
If this resolves the issue, hit me with a Fixya! If not ask me further and I'll try and help you some more.
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