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There is some contradiction in your post. You say you cannot open
notebooks, but CAN see the status "not synched". That means that your
notebook IS opened but only accessible by the ON cache.
At first you should hold your mouse over the notebook tabs at the left
and wait for the pop up window, which says where ON believes the
notebooks are really stored.
Compare that to reality ...
By the way:
If you have lost your original notebook or the paths to them you can
every time create new originals by using "Save as" at a suited place.
After that you should close the newly created notebooks and reopen them
from the new place.
Bernd