if you got drive in 4th only as wammers says theres no drive through lay shaft which is usually caused by poor lubrication,low or contaminated oil, input (constant pinion)and lay shaft teeth are heated to a point that they they can be worn off ,usually does brgs too and case around brgs ,i would get another box as yours is unlikely to be rebuildable as in the case were rear of layshaft shears of in clean break due to weakness in part loosing 5th gear but easily fixable,the loss of all but 4th which is not a gear as such but straight through drive means non through layshaft which is usually catastrophic
