Q: Signature letters have disappeared after installing Yosemite.

I'm not sure what you mean by "Signature letters" but if you are attempting to retrieve mail messages or mailboxes using Time Machine, first launch the Mail app and choose "Enter Time Machine" from the menu bar. You have to do that while the Mail app is open.

Navigate back in time with the "up arrow" and determine if you can find the letters or mailboxes you need to restore.

Are you using iCloud Drive? Generally, signatures are synced across Macs with iCloud. In the previous 3 OS X releases this was with "iCloud documents" and now it's done via "iCloud drive." If you don't have iCloud drive enabled, I would expect signatures, as well as mail rules, to perhaps disappear.

That makes sense. Thanks.

System Preferences/iCloud - make sure you are signed in and then check the box in the right panel.

iCloud Drive FAQ