i had same problem. It happens when flash an wrong code and STM32 can not boot by flash memory any more. You need boot by system, connect Boot0 to Vcc and connect through an usart adapter, Tx to PA10, Rx to PA9, Vcc and Gnd, Just this. Use STM32 cube programmer, option usart, click connect(It works!) and at erase&programming menu , click start programming a correct code. Now You can again use stlink!! with Boot0 to Gnd. That is it !!