1) When yields go up, treasury prices fall. 2) When yields go down, treasury prices rise. 3) The only way you get the basis cost for a treasury back is if you hold to maturity.
When a bank run happens, you (if you are bank) need cash. Lots of it. If you own assets, you have to sell those assets to come up with that cash. SVB had to sell ALL of their notes for less than they bought them for. See #3.
The losses by SVB were realized well before today. This has been going on for a while now. Outside of $250,000 per account type per person, the only additional money folks will get back is whatever a bank sale comes up with, which won't be as much as most folks think, since the core assets were already sold off. If I had to make an educated guess, most startups/investors will lose more than 50% of what they started with. Remember, high interest rates. SVB will NOT be sold at a good price. This is not a market for sellers, it is a market for buyers. Again, sky high interest rates and many of the very investors that could make that sale possible had money at this bank. That means a lower sale price.