This indicates your system had an abnormal exit. (A crash or system shutdown without closing Gramps.)
In the Manage Family Tree dialog, there should be a padlock in the Status column of the row with your Tree name.
Double-clicking the row should bring up a dialog with a Break Lock button. Use that to remove the lock.
The Load Family Tree button should no longer be ghosted. Load the tree normally, make a backup immediately, shutdown Gramps normally and restart it.
This process in the wiki in the Manage Family Trees page section labeled Unlocking a Family Tree
The lock CAN be removed from outside the program manually if absolutely necessary. But it should never be necessary.