This may happen due to connecting to some bad peers or your chain has been corrupted somehow.

First, try simply restarting the wallet then after it's synced up, check if you are on the right chain via article below:

If you are on the right chain but isn't syncing the newer blocks, please follow solution steps outlined in below article to resync cleanly off the network with a new peers.dat.

Also, ensure that your pivx.conf doesn't have any connect= or addnodes= lines unless you know why they are there. 

(pivx.conf is located in the folders listed in below KB article)