12 week old... sleep?

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My 12 week old has been sleeping poorly for the past week... he used to sleep 4-6 sometimes 7 hour stretches.. now we are lucky to get an hour or two 😭😭😭😭 I am beyond exhausted and have no clue what to do!!! I put him down in his bassinet and instantly wakes up.. and if he doesn’t wake up he does in 30 min to an hour.... I do not understand 😢😢😢😢 any help or advice is appreciated