95 D90 5th gear disappeared

Wondering if someone can help me figure out what just happened to my 5th gear. I have a 5 speed manual transmission. Everything was shifting beautifully until this evening while driving home in 5th gear at about 60-65 mph. I accelerated and just got a rapid engine rev with no gear. I slowed and shifted into 4th. The rest of the way home 1st - 4th gears were fine. I tried 5th once again and it just wasn't there. I have searched high and low but haven't found this problem addressed. Any insights into what happened and where to get any parts I might need would be greatly appreciated. I have a Series III that I work on with no major snags but I just got this D90 recently and I am not so sure. Please help if you can.