If you don't know what your boss thinks you could have done better, ask him or her.
You may get useful specific advice -- perhaps regarding customer management, perhaps regarding how to get your company's technical folks to respond more quickly, perhaps something else.
Or the discussion may help your boss understand that, in fact, you did the right things.
Or the discussion may show that the boss was disappointed, but not disappointed in you, and that you shouldn't be taking it so personally.
But in any case, clarifying the statement and seeking advice on how to achieve better results are the next step.
I also have to agree with @Brandin. If nobody in your company can provide support adequate to the customers' needs, the customers are going to go elsewhere. You, or someone, MUST either get that fixes, or accept that your product/service/business is not one that customers can safely bet their business on. Again, this is sometimes a matter of managing the customer's expectations... but if they (for example) have a large e-business site which is offline due to your glitch they may be literally losing millions of dollars in sales every day they're down, and if you promised you could support them when they bought from you then you'd darned well better make every effort to provide that support, including scheduling 24-hour coverage and hauling developers off new work to help solve the problem if that's what it takes.
If the problem is in a vendor, and you can't persuade them that they need to support YOU and your customer that way... you picked the wrong vendor and need to fix that as soon as possible.
Note that realtime support and instantaneous fixes are not the same thing. The customer will (or should) recognize that diagnosis and fix will take a bit of time. They just want that time to be as short as possible. They also will (or should) recognize that problems have different severity levels, and that sometimes they'll have to settle for a workaround rather than a true fix. This is the "customer management" part of the equation.