My custom stm32f103 board held in reset. How can i fix?

[GSay.1]

Hey guys. I am facing with annoying problem. i have designed my own custom stm32 board with stm32f103cbt6 mcu. At the beginning, when i try to burn bootloader via stlink, I successfully did it and i could programmed my custom pcb via usb. the PC13 led was blinking with properly. but suddenly it goes. usb didn`t work and when i tried to burn bootloader again, stlink can not connect and it says the core is held in reset. do you have any idea about it? i have googled it several times, i found some solutions but they didn`t work for mine.

[Uwe Bonnes]

Double check all connections and solder joint. Cross check your debugging procedures against another board. Break down your test in little unit, testing a long chain broken somewhere is hard.

[TDK]

Try to connect under reset with ST-Link. If that doesn't work, check power rails. Sounds like a hardware issue.

[GSay.1]

I already tried to connect under reset mode, nothing change. I ll try your suggestions. Thanks for replies. If it works, i ll post the solution.

[GSay.1]

Update! the problem is absolutely about pcb. i checked all solder stuff and it works.