Never seen that happen unless somebody with access to the account specifically removed it. If you did not do it, and nobody else has access that might have done it, you should change your password right away in case you've been phished. Then see if you can add the account back or not. Maybe at that point it would give you some sort of an error that would explain why, if PayPal actually did remove it. Again, though, I've never seen that happen.
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