1) Make sure you're on the latest release https://github.com/syscoin/blockmarket-desktop-public/releases/
2) Try shutting it down and waiting a full minute before starting up again.
In most cases, this solves the issue.
If this does not solve the issue then:
1) Close the wallet
2) Find your Data Folder:
The Data Folder on Windows is C:\Users\[USERNAME]\AppData\Roaming\Syscoin
The Data Folder on Mac is /Users/[USERNAME]/Library/Application Support/Syscoin
Rename the Data Folder: Sycoin-Old
(This step simply moves the files. This will be your backup)
3) Create a new, empty folder called: Syscoin
4) Copy wallet.dat from the Syscoin-Old folder to the new Syscoin folder.
5) Restart the wallet and wait for it to sync up again.
There is a green bar on the Overview page that indicates syncing progress.