Has the Wifi worked before? If not, see below regarding warranty and customer support. If so, maybe you could try a factory reset and see if this solves your problem. Be sure to back up your data before you do a factory reset as it will erase all your downloaded apps and user data and restore your tablet to factory default condition. Also be sure to fully charge the battery before you reset as well.
The online user manual doesn't give too much detailed information, but usually if you go to the Main menu, then select Settings, then scroll down to Backup and Reset, tap to enter, then select whichever option you want, back up or factory reset and follow the prompts. If this does not resolve your problem, see below regarding warranty and customer support.
Alternatively, you could contact NuVision customer service and ask if they can advise you what can be done to fix your problem. You should check your warranty details and if still valid bear that in mind if you contact NuVision.
Here are their contact details
Email support email@example.com
(800) 890-1288 8:30am - 5:00pm PST Mon-Fri
(866) 490-9945 8:30am - 5:00pm PST Mon-Fri in Spanish