Programming it over ISP implies that the chip is mostly working, since the IDE reads out what it wrote to verify.
When you uploaded the bootloader using ICSP, did you also upload the blink sketch using ICSP, or did you remove the programmer and upload the blink sketch via regular USB?
You have to upload only the bootloader via ICSP and after this upload via USB should work if there is no other problem.
If you can upload a sketch without errors using ICSP or using bootloader/serial your mega is fine. If the blink sketch does not blink the on-board LED you might try uploading a sketch which blinks a different port, like pin 10 for example. Use an external LED or multimeter to check the port.