If the park is still closed due to crowds than no you may not re-enter, you are techincally giving up your space in the park. It depends on what level of closure the parks are at. There are few different levels. for example they may be only letting in Disney resorts guests or it could be totally closed.
Just remember that all the parks have never been closed at the same time due to crowd level. At this time I believe that Disney still guarantees that at least 1 park will be open for resort guests at all times. But if you have been to one park already that day and it is the one that gets closed I don't believe you will able to go to one of the other parks if you don't have a park hopper.
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