It sounds like you have locked the base plan, although that only affects to walls, windows, doors, stairs and objects which have been marked as "part of base plan" so if in that case you still should be able to select normal objects.
Anyway, look at the plan menu, there is a option which says "lock base plan" or "unlock base plan", in the second case just click the option to unlock the plan.
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If a long walk every day were healthy, mailmen would be immortal.
