Hi, most likely your blockchain is corrupted or maybe synced to the old chain beyond the fork height. Please delete your blockchain and resync and you will be good to go, thank you.
THanks for your fast response. But so sorry I don't know how to delete the old chain. Do you have the instruction on how to do it?
Yes, the blockchain files are located in "C:\Users\[yourusername]\AppData\Roaming\digitalnote". Close the client, delete all files in the folder and then restart the client. Make sure that if your wallet file is in that folder that you DON'T delete that. In fact always save a backup of your wallet file!