First, you may be able to solve the problem by updating to the Toad latest version, which just came out a few days ago. You can get version 15.1 here.
If you can't do that....
Are you using Oracle client version 12.1.0.1? If so, upgrade to 12.1.0.2, 12.2, or 18c.
If not, please try resetting user files as described here.
If that solves it, you can being over your connections.xml file to restore your connections.
Also, if that solves it, please zip up and send me your old user files folder (which should now be something like C:\Users\Your_user_name\AppData\Roaming\Quest Software\Toad for Oracle\15.0_bak so that I can try to find what caused the problem and fix it.
Thanks
John