Check the caudex near the soil. Does it feel squishy? If so, it could have root rot. If it feels spongy (distinct difference from squishy!) then it needs water like KatieD said above.
If you are unsure, I would unpot it and check the base and roots. If they look healthy and white with no brown mushy spots, you can repot it, water it in, and it should recover. If there are any brown mushy rotted spots, you will need to completely cut them out, even if it means completely cutting the base off. You should cut away all rot until you only see healthy white tissue remaining in the cut sections. Then let it sit for several days for the fresh cuts to callous over and then repot in well draining soil. Water in, and then only water when it is dry.
Hope this helps, and good luck reviving your adenium!
