Your shutter is stuck. This has the side effect that the mirror will not return to its normal position, which blacks out the viewfinder.
Problems like these are not unusual in older cameras, and should be looked at by someone who's familiar with these old mechanical beasts. Avoid messing with the camera as you might make things worse - shutters are very delicate mechanisms. Keep in mind that spare parts are no longer readily available for these cameras.
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EDIT:
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Some research tells me that stuck shutters are a common problem with this particular camera.
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There appear to be different causes for this problem. Sometimes, it appears to be a matter of "lack of exercise". One suggested solution is a ''gentle'' whack against the palm of the hand :) , another is the use of a fingernail to gently persuade the shutter to close completely - but ''be careful!'' In other cases, the problem seems to be caused by the camera's electrics, specifically, corrosion due to old leaking batteries. Check the inside of the battery compartment for corrosion, if you find any (greenish crud) have the camera cleaned and inspected - the corrosion may have propagated outside the battery compartment.
Your shutter is stuck. This has the side effect that the mirror will not return to its normal position, which blacks out the viewfinder.
Problems like these are not unusual in older cameras, and should be looked at by someone who's familiar with these old mechanical beasts. Avoid messing with the camera as you might make things worse - shutters are very delicate mechanisms. Keep in mind that spare parts are no longer readily available for these cameras.