Yes it happens.
The "investigation" will take an uknown length of time.
Yes, contact your buyer and let them know why you are not shipping, in most cases they will have no idea why PayPal have placed a hold, good idea to give them the impression that you are sure it's not because of anything they did (even if it is).
Wait for a few days and if you hear nothing from PayPal get on the phone with them (PayPal) and demand a resolution (ask for a supervisor if need be).
This situation seems to happen randomly, you're somewhat lucky because often sellers have long since shipped when they get this notice.

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
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