Every bank borrows short term (you can walk up and withdraw your money at any time) but lends long (e.g. mortgages, though SVB writes few of those). The recent management grabbed some very long federal bonds; as rates have risen the resale value of those long term assets (paying a lower interest rate) fell. They can't unwind that position and cover all possible demands.
FDIC will transfer the accounts to another bank, guaranteeing the 250K at least (I believe SVBs own liquid assets could cover that) and may use its own asset base to cover the balance, siezing SVB's other assets and stuffing them into FDIC's piggy bank. It's not like those government bonds won't pay out...eventually.