restart your computer, If that doesn't work just redownload the wallet, yours could be corrupted or changed by a windows update if u use windows.
I have issues from time to time when wallets are updated or changed, make sure you save your password somewhere safe. If no-one else is having the problem it has to be something on your end.
Thanks, for your help. I fixed it and what you post here for someone who may face the same problem some time in the future. Restarting my computer did not work. Redownloading and reinstalling the wallet did not work either at first. After I completely deleted all existing config files (in library folders on Linux OS) and an another reinstall it worked again.