it takes hydraulic fluid to hold it up, and at only 6" up, it should come down all by itself via gravity, unless it has somehow tripped the safety check valve in the cab cylinder base. Did it appear to bounce when it locked up? This may have been possible if you were low on fluid and pumping air right at the top end of the last lift. This would still allow you to push it over the balance point as it doesn't take that much force at that point, but the air moving to the top of the vertical cylinder would be the last to come out as you lower the cab and might cause the safety to lock as it pushes back thru the system and allows rapid passage thru the restrictors in the control valve...
Shift the control to raise and try and raise the cab a little. Sending pressure back into the cylinder base port should reset the safety check, then try to lower it again. but if there is air in the cylinder base it may simply lockup again. If this happens another option may be to pump it up slightly again to release the check, and using a wrench very carefully loosen the hose end connected to the cylinder base port till it just starts to drip fluid and leave it alone and let the cab lower very slowly by itself. the goal here is that when it gets down to the air in the cylinder it will flow out thru the loosened fitting slow enough that it does not trip the safety and continue to lower all the way and push out all the air.
once you get the cab all the way down, and get more fluid, you will want to purge that lift line. set the cab control to lower and pump the lift piston to the fully retracted/down position to get all the air out of the base end of the cylinder. With the cylinder fully retracted, disconnect the hose at the cylinder base end port, select raise on the control valve and pump until you get fluid out the hose, then reconnect the hose. repeat the process at the cab latch hose end then re-attach. Top off the reservoirs and then raise the cab completely...
If you are going to replace it all anyway, you can put about any 10wt oil in it, I like AW-32 because it is available everywhere...