To regain access to your writings, please sign in again to your iCloud account by following the steps in this article:
If you signed back in and still don’t see your texts in Ulysses, please ensure the following settings are correct:
On macOS, go to System Preferences › Apple ID › iCloud (in the left sidebar) and make sure iCloud Drive is enabled. Then, select Options… next to iCloud Drive and check Ulysses.
On iOS/iPadOS, iCloud Drive and Ulysses must be checked in iOS Settings › *your account name* › iCloud.