My 1994 subaru legacy trunk (hatchback) suddenly won't open. It never happened before. Remove the panel where the lock is? From the inside? Ros

no year car told. the now lost free users guide is by year.
made 1994 to present (even 1989 as legacy outback)
no country told at all
sorry cars not the same world wide.

open glove box, read the users guide? oops locked too>?
most cars sold in USA , called wagons
opening the rear gate hatch by accident driving
kids fall the ground, or cargo hits cars behind you. (so is illegall that feature)
and is never called a truck ever, in a wagon
only on sedans. a hatch or 5th door it is named.
In Europe then even call them 5 door cars.
so I am confused.
so all keys lost to the car. and can not drive it
nor open any side doors
and rear hatch is locked and valuables in cargo area to fetch, right?
I say call a locksmith
and pick the lock.
the door lock(mechanism) in the hatch internal, is accessed from the inside of the hatch open.so no why to attack it that way.

call a locksmith,

better to adjust the trunk lock mechanism by trained person.

It appears that the plastic clip on the door controls has broken . need to remove the door cover or trim card and repair/ replace clips . not verry expensive to buy

Button probably bad.
Remove button from dash connect multimeter and set to continuity and press button listening for beep

Most often, access to the trunk is achieved by folding the rear seat down. If not, remove the seat and you'll have access to the trunk area.
If access isn't a problem, get yourself a plastic panel removal tool set from parts store and make sure it's not cold when you remove the panels as they'll be more likely to break from being brittle from the cold. Take your time.

the drivers door lock is the main one in the system and its normally the problem when it goes wrong others are effected,you can remove it lubricate the mechanism and try agin or a replacement is needed,

does this happen when your driving or just opening and closing the trunk? iwould inspect the torsion bars in either case to see if they are properly positioned,they are locate inside the trunk below the package shelf{where 3rd brake is}move the lid up and down and see if you get any squeaks,please dont try to remove them their dangerous and they support the trunk lid

Sounds like the old rusty handle problem. I have found that the handle does not return all the way and it will not let it unlock. Push the handle toward the closed position and try to unlock then open. If this is the problem all you need to do is remove the rust and keep it oiled. Hope this helps and have a great day.

Legacy, right? Can you tilt the backseats' backrests forward? like when you want to transport something big in the trunk... I suppose the sedan has this similarly to the station wagon. 
Use a pocket torch to look for the lock body from inside and there should be a small button or better a lever that you have no clue what it is there for, right? try to push it or pull it from ech side untill it opens. 
It is actually there to prevent transporting people in the trunk against their will(unlike the movies, where it is possible).

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Most of the time these rivits break when you pull them out but sometime you get lucky. They can be bought from subaru at a cheap pruice tag. Just slip a butter knife or something of the sort under the head and pry up. Good Luck. I hope this helps. Don't forget to rate my answer.