If it is just fifth, then it sound as if the sprockets have gone AWOL. This does not mean you need to throw the box away though. With a little time and patience you can replace the fifth gear without removing the gearbox, and if you like, even change the ratio for better MPG on a run.
There is an internet site that explains how to do it. You can practice removing the fifth gear on the car in the scrapyard you remove it from, then put your new found skills to the test on yours.
Also, don't forget to change the gearbox oil as it may be full of iron filings.
