Hi,
Try a hard reset to restore your tablet.
''Be aware that a hard reset will erase all your data and downloaded apps. It will restore the tablet to a factory default condition. It will be like when you first bought it.''
Before you perform a hard reset, ensure that the battery is fully charged and that the external SD Card (if any installed) is removed from the tablet. Also ensure that the charger is disconnected once the battery is fully charged.
Here is a link that shows how to do it. You will have to '''use the 1st Method''' shown as obviously you cannot do the 2nd Method as you cannot access the tablet's settings menus.
http://www.hardreset.info/devices/dell/dell-venue-7-2014/
Hopefully this is of some help.