Red exclamation mark at top-left when booting phone

I was experimenting (more specifically, trying to flash ROMs) with my Samsung Galaxy J1 Ace (rooted). It was all going well until I tried flashing the TWRP recovery mode. I followed all the steps mentioned on various XDA forums, but a red exclamation mark appears at the top left of the screen when I try to turn it on along with the Samsung logo, and the phone just hangs. When I go to the Download mode, it shows "MDM Download Block: 0". I tried flashing, using ODIN, other versions of the TWRP for my phone model, but the same error persists. Is my phone soft-bricked? How do I fix it? I am trying to restore the original stock Samsung firmware now.