Did you happen to push/pull training weeks when editing your calendar?
At the moment, we are unable to accommodate those types of adjustments while retaining the ability to edit your training load down the line. When you attempt to push/pull workouts, we issue a warning that you will lose the ability to edit if you carry out the command, and you must accept to proceed. This warning looks like this:
You should still be able to adjust the plan prior to when you made the adjustment, but afterward, you will not be able to edit in the same way as before. Basically, all of the workouts become disassociated from the plan.
If this is in fact what happened, the best approach would be to re-create your Plan Builder plan using your original start date and all of the same inputs to re-generate the plan. You will first have to delete your current plan, but once you do you, you will be able to automatically pull in all of your events to make the rebuilding process easy. Once you complete this process, your plan will be editable once again.
I hope this clears things up a bit!