My iPhone screen suddenly goes black with circle waiting animation at the center. I forced restart it. And now it can't get past loading screen
I think a restore might be in order. 

First I would plug it on a Mac or PC with an USB cable and do a full encrypted backup. I suppose iTunes will still be able to back it up, but it might require you to authorize its access on the iPhone itself, in which case you'all have to rely on an iCloud Backup restore after the software restore.