Why is my Nissan Altima stuck in 5th gear

Hello, that sounds more like second or third gear, honestly. 5th gear would be pretty high and get you to very high speeds -- but not low ones. In any case, hands on testing to determine what transmission or throttle faults are present is needed. I suspect something is keeping your vehicle in "limp" mode which prevents damage to be caused due to exposure of a component not working properly. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops