I have found that the GUI wallet sticks from time to time and the only solution I have found has been to delete the GUI main folder (the one extracted from the zipped download. Don't delete the Neo file in Win app data/local) and re-install from a fresh download. It does mean syncing again and I have not yet found a "how to" for the bootstrap chain file which is available. It can be a pain sometimes using the GUI wallet especially as the NEON wallet is improving all the time. An easy to find bootstrap file with a guide would make life easier for those of us who like to use the GUI wallet.
I also find that the neo-gui wallet sticks from time to time, the way I found round this was to close and re-open the wallet, that worked most of the time.
There were a couple of times this didn't work and I had to sync from scratch as the blockchain booststrap files recently available (simply a copy of someones chain folder) would not work, but the "chain.acc" bootstrap file, which is now available again, did work.
To get the "chain.acc" boostrap file to work you copy it to the root of the wallet folder after you re-install the wallet, then run the wallet.
Remember to backup your wallet with all you precious NEO in it first.
chain.acc available here
https://github.com/CityOfZion/awesome-neo#blockchain-bootstrap-files