By default, Marvin archives tasks completed more than 6 weeks ago (except tasks in open projects). This is done to improve performance. You can still see all your completed tasks in the Search interface, but you can't edit them unless you enter archive mode.
If you enter archive mode, then all your completed tasks will be loaded and can be edited as normal.
Entering/leaving archive mode
You can enter and exit archive mode in ☰→Settings→Archive:
Configuring archive mode
You can also change how long until completed tasks are archived in ☰→Settings→Archive.