Can't see through viewfinder (it's all black) and

Remove the lens and look inside it. Do you see the mirror? If not, your mirror is stuck in the up position. Its flat up against the focusing screen. If the mirror is down, the problem is in the lens, or you still have the lens cap in place.
Check your manual under troubleshooting.
Try setting the shutter speed dial to the M90 position.
Also be sure the batteries are inserted correctly!!!!

If no change yet, you can try to put the mirror back down yourself. With the lens removed, set the camera on its back. You can use your fingers, but I suggest a toothpick cuz they won't leave fingerprints on the mirror and the toothpick should break before you exert enough pressure to really break something. So just try to return the mirror to the 45 degree angle within the mirror box. Be careful not to damage anything in there, like the shutter or the arms that raise and lower the mirror. Depending on where you live, there may be some corrosion or rust on the mirror linkage. It should move easily. If you feel any roughness when you move the miror, that may be the reason it did not return on its own. Then turn it on, off again. At some point, you may want to have this camera cleaned, lubricated and Adjusted(CLA) - after all, its over 25 yrs old.
If all that did not work, you'll need to send it in. Here are some good choices for repair places...
Garry Airapetov in Niles, ILL
Camera Repairs in Avon, IN.
BTW here's a website with several pages about FG...http://www.mir.com.my/rb/photography/hardwares/classics/emfgfg20/fg/index.htm
copy and paste it into your browser.