During the last couple years, SVB got a ton of deposits, and they didn't have matching loan demand. So they invested the money in bonds. Unfortunately they make a bet that interest rates would stay low, and bought longer duration (~10 year) bonds.
Interest rates have gone up, so the bonds they bought have lost value. They tried this week to fix that by selling part of the portfolio and raising capital, but did it in a ham-handed way and triggered a bank run.