Glyphs sometimes disable LTTR/INK after a crash so you can no longer use skeletons. This article contains steps how to recover to get access to the LTTR/INK tool again.
When Glyphs disables LTTR/INK, you should see the following dialog after you restart Glyphs:
There were lots of fixes and new APIs in recent Glyphs versions focused on LTTR/INK needs so there is a good chance the issue disappears if you upgrade Glyphs. All 2.x/3.x upgrades should be free for all 2.x/3.x licence owners.
If you have issues with upgrading, please contact Glyphs support directly. In order to downgrade from a Cutting Edge version to the latest stable version, uncheck Show Cutting Edge Versions and than download and install Glyphs from their website.
You need to Remove the LTTR/INK plug-in and install it again to unblock it.
Should you experience the crash again, please follow steps in the Report bugs article so we can fix it.