If they blocked you over a fight, give them time to calm down. If this block comes on the heels of an intense argument, give it a few days (or weeks, depending on the severity). It’s possible that the person who blocked you simply needs some room to cool off and that they’ll unblock you once they’re ready to talk. Just give them space.
This is extremely likely to be the case if they’ve blocked and unblocked you before.
If the two of you weren’t fighting, it’s possible they were offended by something you said or did. It’s still best to wait a while for them to cool off.
Find a way to accept this if you cannot change it. When we cannot change the course of events this can plague us. By practicing acceptance and embracing that it cannot be changed, you might be able to let it go and move forward. 

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