Unfortunately, your camera was either stored away after the battery failed, causing mirror lock-up, or during storage the camera shutter operated, there was a long exposure in the dark and the battery failed with the mirror still up.
Either way, you have a locked-up mirror. You've tried the usual cure, a new battery, but as your camera was sitting for so long there's likely been a bit of gumming up in the works. One or both of the following has likely happened:-
- The lubricant films inside the camera have dried out;
- The foam light seals and the foam mirror buffer have decomposed into a sticky, corrosive, black goo.
A professional CLA and foam seal replacement is the official fix (CLA=Clean, Lubricate, Adjust) but the ME Super is very labour-intensive to strip down and there's a high risk of breaking unobtainable parts during the job. The cost of the work far exceeds the camera's value as well. For both these reasons, very few professional repairers will even touch your model and if they do will make it clear that the work is at your own risk and must be paid up-front.
From this point on, you may have gathered that you officially have a write-off suitable for spares only, but don't despair as there are a few things you can try with the lens removed (a lens can also gum up and prevent the body from operating correctly).
First, double check that your batteries are good (new ones can be faulty) are correctly inserted, are clean (they should not be handled by greasy fingers) and that the terminals in the battery compartment are bright and clean
Next, check out the foam issue: there are almost no cameras left from when yours was manufactured with the original foam. It's either been replaced with modern improved material, turned to goo, or the foam crumbled away. In front of the lower edge of the viewfinder screen, there will be a foam mirror buffer which is there to make the camera quieter by deadening the slap of the mirror as it flips upwards. It's very common to find that it's turned to goo and has glued the mirror in the locked-up position. With the camera turned off, gently use your fingernail on the front edge of the mirror to try and prise the mirror down a little. If it doesn't budge, try slipping the tip of a blunt blade (such as a dinner/butter knife) between the mirror and the screen and working it side to side to break the adhesive bond. Be careful to only slip the tip of the blade inas you don't want to scratch the mirror/screen, and you certainly don't want to smear goo over the screen. Don't worry about damaging any remaining foam as it's going to have to be replaced anyway. If this works, you'll be able to pull the mirror down a little and it will spring back up. Now turn the camera on and the mirror should flip down.
If this doesn't work, then try leaving your camera in a dry and warm environment for 24 hours. This can soften any dried out lubricant films sufficiently to free them off. You may also need to slap the camera baseplate firmly down onto a flat surface to free things off and work as many of the manual controls as you can to try and redistribute the internal lubricants. Don't give up on this too soon as it's your last best hope short of stripping down the camera, so also be prepared to give the body some firm taps with something like a rubber mallet (or the heel of your shoe). Remember that you're on a hiding to nothing here as the camera is already a write-off. You may need to repeat this "heat and persuade" method a few times.
If all this has failed then you have two choices: sell the camera as "spares or repairs" for whatever you can get (not much) or obtain a service manual (such as
this free one courtesy of robertstech website) and try to DIY, but be warned: there are a lot of soldered joints to be tackled.
If the repair has worked then pat yourself on the back and order yourself a foam seal and buffer kit from your preferred vendor on sites like eBay and Amazon.
If all has failed then at least you'll have gained valuable experience for your next camera repair attempt and can console yourself by getting another complete replacement SLR outfit for free via FreeCycle or Freegle. As 35mm gear is now largely worthless except for specialist collectable items, many folks prefer to pass the equipment on rather than have the hassle of selling them. I've acquired a wide range of makes and models for free in this way over the last few years and the trick is to keep a regular close eye on the offers and to respond quickly with a personalised and polite reply.
I wish you the best of luck with resolving your issue, please take a moment to rate my reply.