Where are you using Fill & Sign? In the web (Adobe Document Cloud https://cloud.acrobat.com), in Adobe Reader or Acrobat DC, or in an iOS device/app? Also what Operating system and version are you using? If it was in the web which browser and version?
Try restarting whichever application you were using Fill & Sign in and make sure the internet connection is working when opening Fill & Sign. With a bit more information we should be able to help.