Sounds like your program is causing an exception almost immediately and ending up in an endless reboot cycle.
You need to disable you application by sending at+wopen=0 to stop the running application - but you need to do this as soon as the modem boots and before your application causes a reboot. Once you have disabled the application, you can use at+wopen=4 (to erase the application) and at+wopen=3 (to erase any flash objects).
Also, if you are using the adl FLASH api, if you change the size or configuration of your flash object store, you MUST erase the flash using at+wopen=3 before restarting your new application … or you WILL have grief!
BTW, it’s a good idea to have a startup delay in your application so you have a chance to send commands to the device. See here: …hilit=startup+delay#p20148 for some more info…
If all else fails, you’re going to have to build a cable to break out the BOOT pin on the HD15 and use DWLWin to erase the flash application.
Follow the instructions here: …hilit=HD15#p25108 to make such a cable, and follow the link afterwards on how to use DWLWin.