The nut fell of my bass help ?

I have a schecter diamond series elite -5 string bass, i tried playing it today and realized that the action was way too low. The strings were pressured against the neck of the bass. ( It may just be the strings though I havent changed them in a year or so, any advice on that). I decided to take off the strings and when i did the nut came off. Can i just super glue it or leave it like that and put new strings on. Also is there a reason why the action was so low. ( Ihavent used that bass for a good 3 months and it was just sitting there, also its in drop c)